gavin-bc: add meta.changelog

pkgs/tools/misc/gavin-bc/default.nix

···
   meta = {
     homepage = "https://git.gavinhoward.com/gavin/bc";
     description = "Gavin Howard's BC calculator implementation";
+    changelog = "https://git.gavinhoward.com/gavin/bc/raw/tag/${finalAttrs.version}/NEWS.md";
     license = lib.licenses.bsd2;
     maintainers = [ lib.maintainers.AndersonTorres ];
     platforms = lib.platforms.unix;