Move i420 files to the right location

There's also a presubmit check that disallows .. references
in GYP files, which this solves.

BUG=webrtc:5095
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1418753002 .

Cr-Commit-Position: refs/heads/master@{#10371}

webrtc/modules/video_coding/codecs/i420/i420.cc

@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#include "webrtc/modules/video_coding/codecs/i420/main/interface/i420.h"
+#include "webrtc/modules/video_coding/codecs/i420/include/i420.h"
 
 #include <limits>
 #include <string>

webrtc/modules/video_coding/codecs/i420/i420.gypi

@@ -15,7 +15,7 @@
         '<(webrtc_root)/system_wrappers/system_wrappers.gyp:system_wrappers',
       ],
       'sources': [
-        '../interface/i420.h',
+        'include/i420.h',
         'i420.cc',
       ],
     },