update Doxygen config file to include gpu directory in api generation

BUG=skia:2263
R=epoger@google.com

Author: hcm@google.com

Review URL: https://codereview.chromium.org/182253012

git-svn-id: http://skia.googlecode.com/svn/trunk@13705 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/Doxyfile b/Doxyfile
index eafa01a..c472164 100644
--- a/Doxyfile
+++ b/Doxyfile
@@ -16,7 +16,7 @@
 
 # This file only creates documentation for the most important parts of the
 # external-visible API.
-INPUT = include/core include/effects
+INPUT = include/core include/effects include/gpu
 EXTRACT_PRIVATE = YES
 EXTRACT_STATIC = YES