Fix compilation error on FreeBSD 10.0-RELEASE (with clang): change int8_t to
gint.

Change-Id: I7974497a8f50e5d18ec74aa7554f058056a6022c
Reviewed-on: https://code.wireshark.org/review/5115
Petri-Dish: Stephen Fisher <stephenfisher@centurylink.net>
Reviewed-by: Stephen Fisher <stephenfisher@centurylink.net>