Add missing SK_OVERRIDE

Tested by running on clang head + ubuntu 14.04:
GYP_DEFINES=”skia_gpu=0 skia_warnings_as_errors=1" tools/xsan_build thread dm BUILDTYPE=Release
out/Release/dm -v

BUG=skia:3386

Review URL: https://codereview.chromium.org/894833002
diff --git a/gm/hairmodes.cpp b/gm/hairmodes.cpp
index 82a55ea..aa1af03 100644
--- a/gm/hairmodes.cpp
+++ b/gm/hairmodes.cpp
@@ -82,7 +82,7 @@
             return SkString("hairmodes");
         }
 
-        virtual SkISize onISize() { return SkISize::Make(640, 480); }
+        virtual SkISize onISize() SK_OVERRIDE { return SkISize::Make(640, 480); }
 
         void onOnceBeforeDraw() SK_OVERRIDE {
             fBGPaint.setShader(make_bg_shader())->unref();