Fix SampleApp compilation with skia_gpu=0

samplecode/SampleApp.cpp

Index: samplecode/SampleApp.cpp
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 4c930c247ba6cae70960c8148178f204e5e1a3f1..ee2b6e379dc6c7e4a6bbaf2f72f66eed8862845d 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -372,7 +372,11 @@ public:
     }
 
     int numColorSamples() const override {
+#if SK_SUPPORT_GPU
         return fMSAASampleCount;
+#else
+        return 0;
+#endif
     }
 
     int getColorBits() override {