Fix GTK warning in EXTCAP parts

Change-Id: I3a4d9baf3592db1394d1177a19014cbef9140759
Reviewed-on: https://code.wireshark.org/review/5628
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Irene Ruengeler <iruengeler@googlemail.com>