fix broken build on mac due to compiler warning

R=epoger@google.com

Review URL: https://codereview.chromium.org/64423004

git-svn-id: http://skia.googlecode.com/svn/trunk@12175 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/tools/skpdiff/SkImageDiffer.h b/tools/skpdiff/SkImageDiffer.h
index 3b50d57..2c1fa7e 100644
--- a/tools/skpdiff/SkImageDiffer.h
+++ b/tools/skpdiff/SkImageDiffer.h
@@ -19,8 +19,8 @@
     SkImageDiffer();
     virtual ~SkImageDiffer();
 
-    static const double RESULT_CORRECT = 1.0f;
-    static const double RESULT_INCORRECT = 0.0f;
+    static const double RESULT_CORRECT;
+    static const double RESULT_INCORRECT;
 
     /**
      * Gets a unique and descriptive name of this differ