xwm: Fix fd leak in weston_wm_send_data()

The call to source->send(source, mime_type, p[1]); dups the fd, and we have
to close p[1] to not leak it.
diff --git a/src/xwayland/selection.c b/src/xwayland/selection.c
index 76063df..b694477 100644
--- a/src/xwayland/selection.c
+++ b/src/xwayland/selection.c
@@ -447,6 +447,7 @@
 
 	source = seat->selection_data_source;
 	source->send(source, mime_type, p[1]);
+	close(p[1]);
 }
 
 static void