fix(windows-sitl): unblock MSVC and MinGW Windows SITL build

Two compiler-portability fixes that the Windows SITL CI matrix flagged:

* MSVC (px4_parameters.hpp): switch the generated parameters_volatile
  and parameters_readonly arrays from a C-style `T arr[] = {}` to
  `std::array<T, N>`. The C form is a GCC/Clang extension; MSVC
  rejects a zero-size array with C2466 ("cannot allocate an array of
  constant size 0"). px4_sitl has no volatile or readonly params, so
  both arrays land at size 0 and the build fails at line 11302 of
  the generated header. std::array<T, 0> is standard, supports the
  existing range-for in parameters_common.cpp, and keeps the storage
  identical for non-empty boards.

* MinGW (windows/runtime/init.cpp): drop the `extern` keyword from the
  three g_usleep_* globals. With an initializer present, `extern T x =
  v;` is a definition, and GCC under -Wextra -Werror flags it as
  ``initialized and declared 'extern'``. The matching `extern T x;`
  declarations stay in windows_shim/unistd.h; we wrap the definitions
  in `extern "C" { ... }` so the C linkage the inline shim relies on
  is preserved.

Local Ninja+MSVC build of build/px4_sitl_default --target px4
succeeds; bin/px4.exe is produced (10.8 MB).

Signed-off-by: Nuno Marques <n.marques21@hotmail.com>

platforms/posix/src/px4/windows/runtime/init.cpp

@@ -95,15 +95,26 @@ volatile LONG g_px4_session_id = 0;
  * call. They live at file scope because every translation unit that
  * includes <unistd.h> on Windows references them through the inline body
  * of usleep(). */
-extern "C" long g_usleep_pure_spin_us = 5000;
-extern "C" long g_usleep_spin_tail_us = 1000;
+/* GCC's -Werror trips on `extern <type> name = init;` because an
+ * initializer turns the line into a definition, making `extern`
+ * redundant. The matching `extern` declarations live in
+ * platforms/posix/include/windows_shim/unistd.h; here we only need
+ * definitions with external linkage (the default for non-static globals
+ * at namespace scope). Wrapping in `extern "C"` keeps the C linkage that
+ * the inline shim relies on. */
+extern "C" {
+	long g_usleep_pure_spin_us = 5000;
+	long g_usleep_spin_tail_us = 1000;
+}
 /* Floor for the per-thread adaptive spin tail. Initialised by
  * px4_windows_calibrate_usleep_threshold() to the host-measured P95
  * waitable-timer jitter so the controller never collapses below the
  * value we already know is needed to cover the observed long-tail wakes.
  * Defaults to a conservative 700 us when calibration cannot run (e.g.
  * pre-1803 Windows with no high-resolution timer). */
-extern "C" long g_usleep_adaptive_min_tail_us = 700;
+extern "C" {
+	long g_usleep_adaptive_min_tail_us = 700;
+}
 
 /**
  * @brief Auto-tune g_usleep_pure_spin_us against the host's measured

src/lib/parameters/templates/px4_parameters.hpp.jinja

@@ -2,6 +2,7 @@
 
 #include <math.h> // NAN
 
+#include <array>
 #include <stdint.h>
 #include <parameters/param.h>
 
@@ -38,17 +39,21 @@ static constexpr param_type_t parameters_type[] = {
 {%- endfor -%}
 };
 
-static constexpr params parameters_volatile[] = {
+{# Use std::array so that an empty list (size 0) is well-formed on all
+   compilers. A C-style "T arr[] = {}" is a GCC/Clang extension; MSVC
+   rejects it with C2466 ("cannot allocate an array of constant size 0").
+   std::array<T, 0> is standard and supports range-for. #}
+static constexpr std::array<params, {{ volatile_params|length }}> parameters_volatile = { {
 {% for param in volatile_params %}
 	params::{{ param.attrib["name"] }},
 {% endfor %}
-};
+} };
 
-static constexpr params parameters_readonly[] = {
+static constexpr std::array<params, {{ readonly_param_names|length }}> parameters_readonly = { {
 {% for param_name in readonly_param_names %}
 	params::{{ param_name }},
 {% endfor %}
-};
+} };
 
 
 } // namespace px4