gn: Fix cflags usage

R=brettw@chromium.org
TBR=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29519004

Patch from Cem Kocagil <ckocagil@chromium.org>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7198 4adac7df-926f-26a2-2b94-8c16560cd09d