Add on to the dummy.cpp hack that forces wireshark to be linked with a
c++ compiler to automatically create a dummy.cpp file and remove it when
make clean is issued.  This allows make tags to work again since the
file isn't missing.

Change-Id: I17191ad2acc281734fa12da4d347fd3116018f67
Reviewed-on: https://code.wireshark.org/review/5494
Petri-Dish: Stephen Fisher <sfisher@sdf.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Stephen Fisher <sfisher@sdf.org>