mkvtoolnix: patch to build on Darwin & FreeBSD (#311133)

Patch was applied upstream, but no new release was cut yet.
Addresses https://hydra.nixos.org/build/258096907

pkgs/applications/video/mkvtoolnix/default.nix

···
 , cmark
 , docbook_xsl
 , expat
+, fetchpatch2
 , file
 , flac
 , fmt
···
   ++ optionals withGUI [ cmark ]
   ++ optionals stdenv.isLinux [ qtwayland ]
   ++ optionals stdenv.isDarwin [ libiconv ];
+
+  patches = [ (fetchpatch2 {
+    url = "https://gitlab.com/mbunkus/mkvtoolnix/-/commit/7e1bea9527616ab6ab38425e7290579f05dd9bb1.patch";
+    hash = "sha256-9UZrfwrzfKwF8XDzqYnuaDgZws7l1YAb5O1O1+nxo0g=";
+  }) ];
 
   # autoupdate is not needed but it silences a ton of pointless warnings
   postPatch = ''