Cast G_STRUCT_OFFSET to gint to fix mac buildbot

glib casts the result to glong for no apparent reason (has anybody ever defined
a structure of more than 2^32 bytes?) which was causing a whole bunch of useless
64-to-32-bit conversion warnings.

Change-Id: I70305fb3b03332bb876023acdd107eb1e95fea27
Reviewed-on: https://code.wireshark.org/review/3383
Reviewed-by: Evan Huus <eapache@gmail.com>