mupdf: add changelog to meta

pkgs/applications/misc/mupdf/default.nix

···
   meta = with lib; {
     homepage = "https://mupdf.com";
     description = "Lightweight PDF, XPS, and E-book viewer and toolkit written in portable C";
+    changelog = "https://git.ghostscript.com/?p=mupdf.git;a=blob_plain;f=CHANGES;hb=${version}";
     license = licenses.agpl3Plus;
     maintainers = with maintainers; [ vrthra fpletz ];
     platforms = platforms.unix;