semaphore: annotate fallthrough (NFC)

Address `-Werror,-Wimplicit-fallthrough` as identified by clang.  This fixes a
couple of build warnings which are treated as errors.  NFC.

Author: compnerd

src/semaphore.c

@@ -118,7 +118,7 @@ _dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema,
 		}
 		// Fall through and try to undo what the fast path did to
 		// dsema->dsema_value
-	case DISPATCH_TIME_NOW:
+	__attribute__((__fallthrough__)) case DISPATCH_TIME_NOW:
 		orig = dsema->dsema_value;
 		while (orig < 0) {
 			if (os_atomic_cmpxchgvw2o(dsema, dsema_value, orig, orig + 1,
@@ -128,7 +128,7 @@ _dispatch_semaphore_wait_slow(dispatch_semaphore_t dsema,
 		}
 		// Another thread called semaphore_signal().
 		// Fall through and drain the wakeup.
-	case DISPATCH_TIME_FOREVER:
+	__attribute__((__fallthrough__)) case DISPATCH_TIME_FOREVER:
 		_dispatch_sema4_wait(&dsema->dsema_sema);
 		break;
 	}