mark generated version header
mark generated version header

Allows it to pass the license check in chromium.

Cherry picked from libvpx fe4de1f

BUG=chromium:795297

Change-Id: Iebd13f606d61274d185aed075bebd80af5a99f09

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom