G_GNUC_FORMAT encapsulates __attribute((format_arg(...))).

Change-Id: I9513c187ae4ce8b8cf8f6a926072b215e26ec64f
Reviewed-on: https://code.wireshark.org/review/2907
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit eea2e156384f74320deb3f71bb22883d70de7681)
Reviewed-on: https://code.wireshark.org/review/2928