sys/v4l/gstv4lsrc.c: The 'timestamp-offset' property is registered as an int64, so let's use g_value_{set|get}_int64(...

Original commit message from CVS:
* sys/v4l/gstv4lsrc.c: (gst_v4lsrc_set_property),
(gst_v4lsrc_get_property):
The 'timestamp-offset' property is registered as an int64, so
let's use g_value_{set|get}_int64() in our setter and getter
functions (makes it work and fixes warnings with gst-inspect).