Don't build CG on Linux CMake

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1784663002
CQ_EXTRA_TRYBOTS=client.skia.compile:Build-Ubuntu-GCC-x86_64-Release-CMake-Trybot,Build-Mac-Clang-x86_64-Release-CMake-Trybot

Review URL: https://codereview.chromium.org/1784663002

cmake/CMakeLists.txt

@@ -105,6 +105,10 @@ remove_srcs(
     ../src/ports/SkImageGenerator_none.cpp
     ../src/ports/SkTLS_none.cpp)
 
+if (NOT APPLE)
+    remove_srcs(../src/ports/SkImageGeneratorCG.cpp)
+endif()
+
 if (WIN32)
     if(SKIA_GDI)
         remove_srcs(../src/ports/SkFontMgr_win_dw_factory.cpp)

src/ports/SkImageGeneratorCG.cpp

@@ -6,7 +6,6 @@
  */
 
 #include "SkImageGeneratorCG.h"
-#include "SkUnpremultiply.h"
 
 #ifdef SK_BUILD_FOR_MAC
 #include <ApplicationServices/ApplicationServices.h>