Snippets for graphicsmagick 1.3.49 from Nix aarch64-linux

Sourced from https://hydra.nixos.org/log/3rn3ls2prj1pbxhvp1bgk6q60yjmlzsi-graphicsmagick-1.3.49.drv.

• coders/png.c:4326:5: warning: variable 'image_count' set but not used [-Wunused-but-set-variable=]
   4326 |     image_count=0;
        |     ^~~~~~~~~~~