rtems: Create message queue implementation header

Move implementation specific parts of message.h and message.inl into new
header file messageimpl.h.  The message.h contains now only the
application visible API.

cpukit/libmisc/shell/main_wkspaceinfo.c

@@ -20,6 +20,7 @@
 #include <rtems/malloc.h>
 #include <rtems/shell.h>
 #include <rtems/score/protectedheap.h>
+#include <rtems/score/wkspace.h>
 #include "internal.h"
 
 void rtems_shell_print_unified_work_area_message(void)