libvpx.gyp: Fix process_outputs_as_sources usage.

Replace three instances of 'process_output_as_sources' with
'process_outputs_as_sources'. Note that one of these is from
an include.

R=fischman@chromium.org

Review URL: https://codereview.chromium.org/465283003

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libvpx@289332 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
2 files changed