add .cpp files to introspection sources
add .cpp files to introspection sources

see https://github.com/jcupitt/libvips/pull/752

jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips