mark generated rtcd headers
mark generated rtcd headers

Allows them to pass the license check in chromium.

Cherry picked from libvpx 05e6e9a

BUG=chromium:795297

Change-Id: I81c92ae5d6bad2fdbfa17e8cab09f2bdb6fb84d2

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