Fix neon builds for ios.  libyuv_neon macro is not applicable to neon.  its only needed for arm code that is not neon but will link to neon.
BUG=308
TESTED=try bots
R=jzern@chromium.org

Review URL: https://webrtc-codereview.appspot.com/7169005

git-svn-id: http://libyuv.googlecode.com/svn/trunk@973 16f28f9a-4ce2-e073-06de-1de4eb20be90