add empty header for backwards compatibility. BUG=none TESTED=lint passes
add empty header for backwards compatibility.
BUG=none
TESTED=lint passes

Review URL: https://webrtc-codereview.appspot.com/46919004
Vairn/libyuv,chucvv/libyuv,Vairn/libyuv,uditkumawat/libyuv,dexcz/libyuv,uditkumawat/libyuv,yzhikan/libyuv,sathishkumars3/libyuv,dilagurung/libyuv,gigakuma/libyuv,chucvv/libyuv,StopProducing/libyuv,chucvv/libyuv,mosterta/libyuv,dexcz/libyuv,sathishkumars3/libyuv,gigakuma/libyuv,dilagurung/libyuv,StopProducing/libyuv,hyper123/libyuv,yzhikan/libyuv,RockySteveJobs/libyuv,dexcz/libyuv,hyper123/libyuv,RockySteveJobs/libyuv,dilagurung/libyuv,StopProducing/libyuv,mosterta/libyuv,sathishkumars3/libyuv,mosterta/libyuv,gigakuma/libyuv,RockySteveJobs/libyuv,Vairn/libyuv,hyper123/libyuv,uditkumawat/libyuv,yzhikan/libyuv