PIDL: fix -Wmissing-prototype

Change-Id: I4cae47450e8026b10bd373828f235184560e0a99
Reviewed-on: https://code.wireshark.org/review/4308
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>