mark generated rtcd headers

Allows them to pass the license check in chromium.

BUG=chromium:98319

Change-Id: Ib37bf45bdac8cf1edc62037dea17b734a5e37fa7

build/make/rtcd.pl

@@ -200,6 +200,7 @@ sub filter {
 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}