mastodon: correctly apply patches

`inherit patches` would be more idiomatic but this is what the `update.sh` script produces.

Fixes a regression introduced in #558824.

(cherry picked from commit 9256e7614a)

pkgs/by-name/ma/mastodon/source.nix

@@ -19,5 +19,5 @@ applyPatches {
       yarnMissingHashes = ./missing-hashes.json;
     };
   };
-  patches = [ ];
+  patches = patches ++ [ ];
 }