LibPthread: Define SEM_FAILED in semaphore.h

This makes the _multiprocessing module from the Python port build. :^)

Userland/Libraries/LibPthread/semaphore.h

@@ -28,6 +28,7 @@ int sem_unlink(const char*);
 int sem_wait(sem_t*);
 int sem_timedwait(sem_t*, const struct timespec* abstime);
 
+#define SEM_FAILED ((sem_t*)0)
 #define SEM_VALUE_MAX INT_MAX
 
 __END_DECLS