gl-renderer: fix the build warning

warning: no previous prototype for ‘gl_renderer_sync_post’
[-Wmissing-prototypes]

Upstream-Status: Inappropriate [i.MX-specific]

Signed-off-by: Yong Gan <yong.gan@nxp.com>
diff --git a/libweston/gl-renderer.c b/libweston/gl-renderer.c
index 2177e65..1f8fb85 100644
--- a/libweston/gl-renderer.c
+++ b/libweston/gl-renderer.c
@@ -3784,7 +3784,8 @@
 	return 0;
 }
 
-int gl_renderer_sync_post(struct weston_compositor *ec)
+static int
+gl_renderer_sync_post(struct weston_compositor * ec)
 {
 	struct gl_renderer *gr = get_renderer(ec);
 	return gr->sync_post;