qt5.qtmacextras: include cf-private on darwin

Undefined symbols for architecture x86_64:
      "_OBJC_CLASS_$_NSData", referenced from:
          objc-class-ref in qmacfunctions.o
      "_OBJC_CLASS_$_NSMutableArray", referenced from:
          objc-class-ref in qmactoolbar.o
          objc-class-ref in qmactoolbardelegate.o
    ld: symbol(s) not found for architecture x86_64

pkgs/development/libraries/qt-5/modules/qtmacextras.nix

@@ -1,9 +1,11 @@
-{ qtModule, qtbase, lib }:
+{ stdenv, qtModule, qtbase, cf-private }:
 
 qtModule {
   name = "qtmacextras";
-  qtInputs = [ qtbase ];
-  meta = with lib; {
+  qtInputs = [ qtbase ]
+    # Needed for _OBJC_CLASS_$_NSData symbols.
+    ++ stdenv.lib.optional stdenv.isDarwin cf-private;
+  meta = with stdenv.lib; {
     maintainers = with maintainers; [ periklis ];
     platforms = platforms.darwin;
   };