score: Inhibit a scheduler change without SMP

The POSIX sporadic server takes the real priority node of a thread out
of the priority aggregation while the thread runs at the low priority.
_Scheduler_Set() extracts that node without a check.  It skips the
extract where a scheduler change inhibitor is present, and only an SMP
build had one.  A uniprocessor build with the POSIX API therefore
extracted a node which sits in no tree.  A debug build ended psx09 with
a failed assertion in _RBTree_Extract() and a release build broke the
priority aggregation of the thread.

The sporadic server is a property of the POSIX API alone.  Enable the
scheduler change inhibitors wherever the POSIX API is enabled.
rtems_task_set_scheduler() then returns RTEMS_RESOURCE_IN_USE for a
sporadic server thread at both priorities, as it does in an SMP build.

Closes #5745.

Assisted-by: Claude:claude-opus-5 claude-code
Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

cpukit/include/rtems/score/thread.h

@@ -112,7 +112,7 @@ extern "C" {
 #define RTEMS_SCORE_THREAD_REAL_PRIORITY_MAY_BE_INACTIVE
 #endif
 
-#if defined( RTEMS_POSIX_API ) && defined( RTEMS_SMP )
+#if defined( RTEMS_POSIX_API )
 /**
  * @brief This define enables support to inhibit scheduler changes.
  *