Fix SampleApp compilation with skia_gpu=0

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2347593002

Review-Url: https://codereview.chromium.org/2347593002
diff --git a/samplecode/SampleApp.cpp b/samplecode/SampleApp.cpp
index 4c930c2..ee2b6e3 100644
--- a/samplecode/SampleApp.cpp
+++ b/samplecode/SampleApp.cpp
@@ -372,7 +372,11 @@
     }
 
     int numColorSamples() const override {
+#if SK_SUPPORT_GPU
         return fMSAASampleCount;
+#else
+        return 0;
+#endif
     }
 
     int getColorBits() override {