graphics/photoflow: prepare for glib 2.68.3

Remove last remnant of cimg_funcs.h

The cimg functionality in graphics/vips was removed in 2014 in in

https://github.com/libvips/libvips/commit/927b2d795448eb676d4c1680f07b54d61bb4dd7c

The header was completely removed in

https://github.com/libvips/libvips/commit/221a5f7e8bfd11797f1bef5a29a928bc0f03e311

PR: 256200