graphite2: enable build on Darwin

This builds on Darwin without any modifications, so it has been added to
the platforms array

pkgs/development/libraries/silgraphite/graphite2.nix

@@ -14,6 +14,6 @@ stdenv.mkDerivation rec {
   meta = {
     description = "An advanced font engine";
     maintainers = [ stdenv.lib.maintainers.raskin ];
-    hydraPlatforms = stdenv.lib.platforms.linux;
+    hydraPlatforms = stdenv.lib.platforms.unix;
   };
 }