Compile with -Wcast-function-type on Travis
Compile with -Wcast-function-type on Travis

See:
https://github.com/libvips/libvips/pull/1697#issuecomment-657602404

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