mark generated version header

Allows it to pass the license check in chromium.

Cherry picked from libvpx fe4de1f

BUG=chromium:795297

Change-Id: Iebd13f606d61274d185aed075bebd80af5a99f09

build/make/version.sh

diff --git a/build/make/version.sh b/build/make/version.sh
index 2a7090e..0dfd235 100755
--- a/build/make/version.sh
+++ b/build/make/version.sh
@@ -61,6 +61,7 @@
     echo "${changelog_version}${git_version_id}" > $$.tmp
 else
     cat<<EOF>$$.tmp
+// This file is generated. Do not edit.
 #define VERSION_MAJOR  $major_version
 #define VERSION_MINOR  $minor_version
 #define VERSION_PATCH  $patch_version