Export effects classes with SK_API.

So this works in shared_library build in chromium.

This issue was raised up in the following chromium-dev thread:
https://groups.google.com/a/chromium.org/forum/?hl=en&fromgroups=#!topic/chromium-dev/Z6qzEw8MwVY

R=epoger@google.com,tomhudson@google.com

Review URL: https://codereview.appspot.com/6601065

git-svn-id: http://skia.googlecode.com/svn/trunk@5914 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/include/effects/SkPorterDuff.h b/include/effects/SkPorterDuff.h
index 8308f3c..1bba171 100644
--- a/include/effects/SkPorterDuff.h
+++ b/include/effects/SkPorterDuff.h
@@ -1,4 +1,3 @@
-
 /*
  * Copyright 2006 The Android Open Source Project
  *
@@ -6,7 +5,6 @@
  * found in the LICENSE file.
  */
 
-
 #ifndef SkPorterDuff_DEFINED
 #define SkPorterDuff_DEFINED
 
@@ -17,7 +15,7 @@
 
 /** DEPRECATED - use SkXfermode::Mode instead
  */
-class SkPorterDuff {
+class SK_API SkPorterDuff {
 public:
     /** List of predefined xfermodes. In general, the algebra for the modes
         uses the following symbols:
@@ -81,4 +79,3 @@
 };
 
 #endif
-