Add SK_API to null canvas create method

git-svn-id: http://skia.googlecode.com/svn/trunk@4221 2bbb7eff-a529-9590-31e7-b0007b416f81

include/utils/SkNullCanvas.h

@@ -15,6 +15,6 @@ class SkCanvas;
 /**
  * Creates a canvas that draws nothing. This is useful for performance testing.
  */
-SkCanvas* SkCreateNullCanvas();
+SK_API SkCanvas* SkCreateNullCanvas();
 
 #endif