fix omission of non-stub pthread_mutexattr_getprotocol

this change should have been made when priority inheritance mutex
support was added. if priority protection is also added at some point
the implementation will need to change and will probably no longer be
a simple bit shuffling.

src/thread/pthread_attr_get.c

@@ -70,7 +70,7 @@ int pthread_condattr_getpshared(const pthread_condattr_t *restrict a, int *restr
 
 int pthread_mutexattr_getprotocol(const pthread_mutexattr_t *restrict a, int *restrict protocol)
 {
-	*protocol = PTHREAD_PRIO_NONE;
+	*protocol = a->__attr / 8U % 2;
 	return 0;
 }
 int pthread_mutexattr_getpshared(const pthread_mutexattr_t *restrict a, int *restrict pshared)