mark generated rtcd headers

Allows them to pass the license check in chromium.

Cherry picked from libvpx 05e6e9a

BUG=chromium:795297

Change-Id: I81c92ae5d6bad2fdbfa17e8cab09f2bdb6fb84d2
diff --git a/build/make/rtcd.pl b/build/make/rtcd.pl
index d57e0df..5c6106c 100755
--- a/build/make/rtcd.pl
+++ b/build/make/rtcd.pl
@@ -214,6 +214,7 @@
 sub common_top() {
   my $include_guard = uc($opts{sym})."_H_";
   print <<EOF;
+// This file is generated. Do not edit.
 #ifndef ${include_guard}
 #define ${include_guard}