Fix warning: no previous prototype for function ... [-Wmissing-prototypes]

warning coming after g055e4768ece and g421913b721

Change-Id: I5caeeb89c559faef8007f2946c68c7172b75dfa8
Reviewed-on: https://code.wireshark.org/review/5250
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>