Add SK_API to SkString for hclam@chromium.org

TBR=reed@google.com

Review URL: https://codereview.appspot.com/6737047

git-svn-id: http://skia.googlecode.com/svn/trunk@6008 2bbb7eff-a529-9590-31e7-b0007b416f81

include/core/SkString.h

@@ -64,7 +64,7 @@ char* SkStrAppendFixed(char buffer[], SkFixed);
     counting to make string assignments and copies very fast
     with no extra RAM cost. Assumes UTF8 encoding.
 */
-class SkString {
+class SK_API SkString {
 public:
                 SkString();
     explicit    SkString(size_t len);