dmd: mark unbroken (#119647)

It is building fine locally, tested by myself and @SuperSandro2000 (who had added the broken tag).

Should this be tested on hydra before removing it? I don't know if that is even possible.

pkgs/development/compilers/dmd/default.nix

···
     maintainers = with maintainers; [ ThomasMader lionello ];
     platforms = [ "x86_64-linux" "i686-linux" "x86_64-darwin" ];
     # many tests are failing
-    broken = true;
   };
 }