[video_player] fix type mismatch

[makotosato-at]

The following error occurs when building:

gst_video_player.cc:135:8: error: no matching function for call to
'gst_element_query_duration'
  if (!gst_element_query_duration(gst_.pipeline, fmt, &duration_msec)) {
       ^~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/gstreamer-1.0/gst/gstutils.h:1057:25: note: candidate function not viable:
no known conversion from 'int64_t *' (aka 'long long *') to 'gint64 *' (aka 'long *') for
3rd argument
gboolean                gst_element_query_duration      (GstElement *element, GstFormat
format, gint64 *duration);
                        ^
1 error generated.

So, I fix it.