restore [6] for setPrefConfigTable, accidentally changed with A1 CL

BUG=

Review URL: https://codereview.chromium.org/99963002

git-svn-id: http://skia.googlecode.com/svn/trunk/include@12433 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/core/SkImageDecoder.h b/core/SkImageDecoder.h
index 5745dbd..a7e3646 100644
--- a/core/SkImageDecoder.h
+++ b/core/SkImageDecoder.h
@@ -186,7 +186,7 @@
             src: 32/24,   no-alpha  -> 4
             src: 32/24,   yes-alpha -> 5
      */
-    void setPrefConfigTable(const SkBitmap::Config pref[5]);
+    void setPrefConfigTable(const SkBitmap::Config pref[6]);
 
     /**
      *  Optional table describing the caller's preferred config based on