compositor: Return 0 on success

Add missing return statement.
diff --git a/src/data-device.c b/src/data-device.c
index ec3df33..44ec989 100644
--- a/src/data-device.c
+++ b/src/data-device.c
@@ -393,6 +393,8 @@
 	weston_pointer_set_focus(seat->pointer, NULL,
 				 wl_fixed_from_int(0), wl_fixed_from_int(0));
 	weston_pointer_start_grab(seat->pointer, &drag->grab);
+
+	return 0;
 }
 
 static void