libXres: enable cross-compilation

- adds utilmacros
- set null malloc flag

pkgs/servers/x11/xorg/overrides.nix

@@ -256,6 +256,9 @@ self: super:
 
   libXres = super.libXres.overrideAttrs (attrs: {
     outputs = [ "out" "dev" "devdoc" ];
+    buildInputs = with self; attrs.buildInputs ++ [ utilmacros ];
+    configureFlags = attrs.configureFlags or []
+      ++ malloc0ReturnsNullCrossFlag;
   });
 
   libXScrnSaver = super.libXScrnSaver.overrideAttrs (attrs: {