Don't build CG on Linux CMake

src/ports/SkImageGeneratorCG.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkImageGeneratorCG.h"
-#include "SkUnpremultiply.h"
 
 #ifdef SK_BUILD_FOR_MAC
 #include <ApplicationServices/ApplicationServices.h>
 #endif
 
 #ifdef SK_BUILD_FOR_IOS
 #include <CoreGraphics/CoreGraphics.h>
 #include <ImageIO/ImageIO.h>
 #include <MobileCoreServices/MobileCoreServices.h>
 #endif
@@ skipping 94 matching lines @@
     //            kGray_8_SkColorType
     //            kIndex_8_SkColorType
     //        Additionally, it would be interesting to compare the performance
     //        of SkSwizzler with CG's built in swizzler.
     if (!SkCopyPixelsFromCGImage(info, rowBytes, pixels, image)) {
         return false;
     }
 
     return true;
 }