ping all classes in dzsave
ping all classes in dzsave

fixes a race in class build on many-core machines, see

https://github.com/jcupitt/libvips/issues/64

mcanthony/libvips,lovell/libvips,leiyangyou/libvips,lovell/libvips,mcanthony/libvips,jcupitt/libvips,mcanthony/libvips,wishabi/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,mcanthony/libvips,leiyangyou/libvips,leiyangyou/libvips,jcupitt/libvips,wishabi/libvips,wishabi/libvips,leiyangyou/libvips,wishabi/libvips