mpci: Hide implementation details

This improves the standard compatibility of API headers.  It fixes
errors like this if RTEMS_MULTIPROCESSING is enabled:

cpukit/include/rtems/score/processormask.h: In function 'uint32_t _Processor_mask_Find_last_set(const Processor_mask*)':
cpukit/include/rtems/score/processormask.h:339:21: error: 'flsl' was not declared in this scope
  339 |   return (uint32_t) __BIT_FLS( CPU_MAXIMUM_PROCESSORS, a );
      |                     ^~~~~~~~~

cpukit/include/rtems/rtems/partdata.h

@@ -116,6 +116,8 @@ typedef struct {
 extern Objects_Information _Partition_Information;
 
 #if defined(RTEMS_MULTIPROCESSING)
+struct _Thread_Control;
+
 /**
  * @brief Sends the extract proxy request.
  *
@@ -126,7 +128,7 @@ extern Objects_Information _Partition_Information;
  * @param id is the partition identifier.
  */
 void _Partition_MP_Send_extract_proxy (
-  Thread_Control *the_thread,
+  struct _Thread_Control *the_thread,
   Objects_Id              id
 );
 #endif

cpukit/include/rtems/score/mpci.h

@@ -40,9 +40,6 @@
 #define _RTEMS_SCORE_MPCI_H
 
 #include <rtems/score/mppkt.h>
-#include <rtems/score/thread.h>
-#include <rtems/score/threadq.h>
-#include <rtems/score/watchdog.h>
 
 #ifdef __cplusplus
 extern "C" {

cpukit/include/rtems/score/mpciimpl.h

@@ -39,6 +39,9 @@
 #define _RTEMS_SCORE_MPCIIMPL_H
 
 #include <rtems/score/mpci.h>
+#include <rtems/score/thread.h>
+#include <rtems/score/threadq.h>
+#include <rtems/score/watchdog.h>
 #include <rtems/score/status.h>
 
 #ifdef __cplusplus