fix(posix): keep shim header nullptr behind __cplusplus guard

The earlier `style(posix): use nullptr in Windows shim headers` change
silenced clang-tidy hicpp-use-nullptr by writing `nullptr` directly into
the shim header bodies. That worked from C++ TUs but broke Windows SITL
on every C TU that includes those shims (e.g. sitl_led.c via
px4_platform_common/log.h pulling in time.h):

  windows_shim/time.h(109): error C2065: 'nullptr': undeclared identifier
  windows_shim/time.h:109:41: error: 'nullptr' undeclared

`nullptr` is a C++ keyword and only became a C23 keyword; pre-C23 C
treats it as an undeclared identifier. Add a small `__cplusplus`-guarded
fall-back macro at the top of each affected shim header so the inline
bodies stay valid from C while preserving the lint-clean spelling for
C++ callers. No behavior change.

Affected headers:
- platforms/posix/include/windows_shim/time.h
- platforms/posix/include/windows_shim/libgen.h
- platforms/posix/include/windows_shim/dirent.h
- platforms/posix/include/windows_shim/poll.h

Verified locally with MSVC by building drivers_board (sitl_led.c
compiles clean).

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

platforms/posix/include/windows_shim/dirent.h

@@ -51,6 +51,13 @@
 #include <string.h>
 #include <errno.h>
 
+/* This header is consumed from both C and C++ translation units. Provide a
+ * fall-back macro so the inline shim bodies below can spell `nullptr` while
+ * remaining valid in C, where `nullptr` is reserved before C23. */
+#if !defined(__cplusplus) && !defined(nullptr)
+#  define nullptr NULL
+#endif
+
 #ifndef NAME_MAX
 #define NAME_MAX 260
 #endif

platforms/posix/include/windows_shim/libgen.h

@@ -42,6 +42,13 @@
 
 #include <string.h>
 
+/* This header is consumed from both C and C++ translation units. Provide a
+ * fall-back macro so the inline shim bodies below can spell `nullptr` while
+ * remaining valid in C, where `nullptr` is reserved before C23. */
+#if !defined(__cplusplus) && !defined(nullptr)
+#  define nullptr NULL
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

platforms/posix/include/windows_shim/poll.h

@@ -50,6 +50,13 @@
 #include <winsock2.h>
 #include <windows.h>
 
+/* This header is consumed from both C and C++ translation units. Provide a
+ * fall-back macro so the inline shim bodies below can spell `nullptr` while
+ * remaining valid in C, where `nullptr` is reserved before C23. */
+#if !defined(__cplusplus) && !defined(nullptr)
+#  define nullptr NULL
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif

platforms/posix/include/windows_shim/time.h

@@ -42,6 +42,14 @@
 
 #ifdef _WIN32
 
+/* This header is consumed from both C and C++ translation units (e.g.
+ * sitl_led.c on Windows SITL). Use a fall-back macro so the inline shim
+ * bodies below can spell `nullptr` while remaining valid in C, where
+ * `nullptr` is reserved before C23. */
+#if !defined(__cplusplus) && !defined(nullptr)
+#  define nullptr NULL
+#endif
+
 #if defined(_MSC_VER) && !defined(__clang__)
 #if defined(__has_include)
 #  if __has_include(<../ucrt/time.h>)