Put a C comment in dummy.cpp so the user knows where dummy.cpp came from
if it is ever created during the build process.

Change-Id: Ib3131f63c965ae243f25f98f6209e049323d8360
Reviewed-on: https://code.wireshark.org/review/6030
Petri-Dish: Stephen Fisher <sfisher@sdf.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stephen Fisher <sfisher@sdf.org>