remove double including of libpng decoder

R=bungeman@google.com

Author: zachr@google.com

Review URL: https://chromiumcodereview.appspot.com/17408002

git-svn-id: http://skia.googlecode.com/svn/trunk@9666 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gm/gmmain.cpp b/gm/gmmain.cpp
index a5674a5..443a26b 100644
--- a/gm/gmmain.cpp
+++ b/gm/gmmain.cpp
@@ -24,6 +24,7 @@
 #include "SkDeferredCanvas.h"
 #include "SkDevice.h"
 #include "SkDrawFilter.h"
+#include "SkForceLinking.h"
 #include "SkGPipe.h"
 #include "SkGraphics.h"
 #include "SkImageDecoder.h"
@@ -37,6 +38,8 @@
 #include "SkTileGridPicture.h"
 #include "SamplePipeControllers.h"
 
+__SK_FORCE_IMAGE_DECODER_LINKING;
+
 #ifdef SK_BUILD_FOR_WIN
     // json includes xlocale which generates warning 4530 because we're compiling without
     // exceptions; see https://code.google.com/p/skia/issues/detail?id=1067