skip isac_neon if neon is not supported

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/39909004

Patch from Mostyn Bramley-Moore <mostynb@opera.com>.

Cr-Commit-Position: refs/heads/master@{#8610}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8610 4adac7df-926f-26a2-2b94-8c16560cd09d
1 file changed