fix arg type for TIFFTAG_SUBIFD
fix arg type for TIFFTAG_SUBIFD

We were passing int* when TIFFGetField wanted unit16, so subifd counts
could sometimes be wrong.

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