Ensure that SkDebugf is exported from the library.

This was caused by the change to always call SkDebugf() in SkASSERT(). There are many headers in Skia that call SkASSERT and so we were seeing linking issues in Gecko where we were using Skia across DLL boundaries.

R=reed@google.com, bsalomon@google.com, reed1
BUG=skia:

Author: george@mozilla.com

Review URL: https://codereview.chromium.org/438783002
diff --git a/include/core/SkTypes.h b/include/core/SkTypes.h
index e264f48..4d590c2 100644
--- a/include/core/SkTypes.h
+++ b/include/core/SkTypes.h
@@ -89,7 +89,7 @@
 #define SK_INIT_TO_AVOID_WARNING    = 0
 
 #ifndef SkDebugf
-    void SkDebugf(const char format[], ...);
+    SK_API void SkDebugf(const char format[], ...);
 #endif
 
 #ifdef SK_DEBUG