include planar functions for I420Rect()

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}

webrtc/common_video/video_frame_buffer.cc

@@ -15,6 +15,7 @@
 #include "webrtc/base/checks.h"
 #include "webrtc/base/keep_ref_until_done.h"
 #include "libyuv/convert.h"
+#include "libyuv/planar_functions.h"
 #include "libyuv/scale.h"
 
 // Aligning pointer to 64 bytes for improved performance, e.g. use SIMD.