[mutter] screen-cast/src: Fix signedness of timestamp field

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 7adc24d3a681067e96ca4905716fb976f0125068
Author: Jonas Ådahl <jadahl gmail com>
Date:   Fri Jul 3 23:50:28 2020 +0200

    screen-cast/src: Fix signedness of timestamp field
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1361

 src/backends/meta-screen-cast-stream-src.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/backends/meta-screen-cast-stream-src.c b/src/backends/meta-screen-cast-stream-src.c
index 7fe603b746..53bfa47b3d 100644
--- a/src/backends/meta-screen-cast-stream-src.c
+++ b/src/backends/meta-screen-cast-stream-src.c
@@ -91,7 +91,7 @@ typedef struct _MetaScreenCastStreamSrcPrivate
   struct spa_video_info_raw video_format;
   int video_stride;
 
-  uint64_t last_frame_timestamp_us;
+  int64_t last_frame_timestamp_us;
 
   GHashTable *dmabuf_handles;