opencv: add cf-private on darwin

Undefined symbols for architecture x86_64:
"_NSDefaultRunLoopMode", referenced from:
_cvWaitKey in window_cocoa.mm.o
"_OBJC_CLASS_$_NSDate", referenced from:
objc-class-ref in window_cocoa.mm.o
objc-class-ref in cap_qtkit.mm.o
"_OBJC_CLASS_$_NSDictionary", referenced from:
objc-class-ref in window_cocoa.mm.o
objc-class-ref in cap_qtkit.mm.o
"_OBJC_CLASS_$_NSMutableDictionary", referenced from:
objc-class-ref in window_cocoa.mm.o
"_OBJC_CLASS_$_NSRunLoop", referenced from:
objc-class-ref in cap_qtkit.mm.o
"_OBJC_CLASS_$_NSTimer", referenced from:
objc-class-ref in cap_qtkit.mm.o
ld: symbol(s) not found for architecture x86_64

pkgs/development/libraries/opencv/default.nix

···
 , enableFfmpeg ? false, ffmpeg
 , enableGStreamer ? false, gst_all_1
 , enableEigen ? true, eigen
-, darwin
+, cf-private, Cocoa, QTKit
 }:
 
 let
···
     ++ lib.optional enableFfmpeg ffmpeg
     ++ lib.optionals enableGStreamer (with gst_all_1; [ gstreamer gst-plugins-base ])
     ++ lib.optional enableEigen eigen
-    ++ lib.optionals stdenv.isDarwin (with darwin.apple_sdk.frameworks; [ Cocoa QTKit ])
+    ++ lib.optionals stdenv.isDarwin [ Cocoa QTKit cf-private /* For NSDefaultRunLoopMode */ ]
     ;
 
   propagatedBuildInputs = lib.optional enablePython pythonPackages.numpy;

pkgs/top-level/all-packages.nix

···
 
   opencv = callPackage ../development/libraries/opencv {
     ffmpeg = ffmpeg_2;
+    inherit (darwin) cf-private;
+    inherit (darwin.apple_sdk.frameworks) Cocoa QTKit;
   };
 
   opencv3 = callPackage ../development/libraries/opencv/3.x.nix {