nixos/gerrit: Add Felix Singer as maintainer

I'm already a maintainer of the Gerrit package and I've done various
work on the Gerrit module. So it makes sense to be a maintainer there as
well.

Signed-off-by: Felix Singer <felixsinger@posteo.net>

nixos/modules/services/web-apps/gerrit.nix

@@ -261,6 +261,7 @@ in
   meta.maintainers = with lib.maintainers; [
     edef
     zimbatm
+    felixsinger
   ];
   # uses attributes of the linked package
   meta.buildDocsInSandbox = false;