The prototype for I420Rect is in libyuv/planar_functions.h
In the past the header was included by convert.h but in future versions
the headers dont include each other so its important to include the
specific header needed.
R=marpan@webrtc.org
BUG=webrtc:6091
Review-Url: https://codereview.webrtc.org/2130153005
Cr-Commit-Position: refs/heads/master@{#13421}