CMake: Bundle our plugins.

plugins/*/CMakeLists.txt has a lot of repitition. We might want to
create a module or include file to simplify things.

Change-Id: Iadd453c286a4127beacd80edf6dc200aa9148852
Reviewed-on: https://code.wireshark.org/review/4582
Reviewed-by: Gerald Combs <gerald@wireshark.org>