testsuites/unit/tc-score-msgq.c: Fix enum conversion warnings

The GCC warning -Wenum-conversion flagged the call to
_Core_message_queue_Submit() as returning a Status_Control
but the caller assigned it to a variable of type rtems_status_code.

testsuites/unit/tc-score-msgq.c

@@ -163,7 +163,7 @@ static rtems_status_code SubmitMessage(
   unsigned int posix_piority
 )
 {
-  rtems_status_code status;
+  Status_Control status;
   Thread_queue_Context queue_context;
   Message_queue_Control *the_message_queue;