Use target_include_directories instead of include_directories in a few
places as recommended at
https://cmake.org/cmake/help/latest/command/include_directories.html
Doing so lets us mark a bunch of dependency includes SYSTEM PRIVATE, in
particular LIBXML2_INCLUDE_DIRS. On macOS this keeps us from triggering
the nullability warnings described at
https://www.wireshark.org/lists/wireshark-dev/202004/msg00056.html
(This might also keep the Visual Studio code analyzer from complaining
about various Qt headers, but I haven't tested this.)