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
1 file changed