With CMake, define WANT_PACKET_EDITOR in config.h.

Just as we do with autotools and nmake, define it in config.h, so it's
defined the same for every single file, rather than defining it in one
particular directory's Makefiles, so that it's defined for some code but
not all.

Change-Id: Ic41cd10f2b502e0be76c134cbeaad94dff7623f8
Reviewed-on: https://code.wireshark.org/review/4891
Reviewed-by: Guy Harris <guy@alum.mit.edu>