omx: also reset nTimeStamp when re-using buffers

Some OMX implementations may check if the timestamp of the output buffers
they receive is actually not set.

https://bugzilla.gnome.org/show_bug.cgi?id=788711

omx/gstomx.c

@@ -231,6 +231,7 @@ gst_omx_buffer_reset (GstOMXBuffer * buf)
   buf->omx_buf->nFlags = 0;
   buf->omx_buf->nOffset = 0;
   buf->omx_buf->nFilledLen = 0;
+  buf->omx_buf->nTimeStamp = 0;
 }
 
 /* NOTE: Call with comp->lock, comp->messages_lock will be used */