Build fix fo windows only

Build fix

git-svn-id: http://skia.googlecode.com/svn/trunk/src@11828 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkRefCnt.cpp b/core/SkRefCnt.cpp
index cf8923d..e9eaea9 100644
--- a/core/SkRefCnt.cpp
+++ b/core/SkRefCnt.cpp
@@ -12,7 +12,8 @@
 SK_DEFINE_INST_COUNT(SkRefCnt)
 SK_DEFINE_INST_COUNT(SkWeakRefCnt)
 
+#ifdef SK_BUILD_FOR_WIN
 SkRefCnt::SkRefCnt(const SkRefCnt&) { }
-
-SkRefCnt::operator=(const SkRefCnt&) { }
+SkRefCnt& SkRefCnt::operator=(const SkRefCnt&) { return *this; }
+#endif