Start to implicitly include \n in rc_header_format.

Certain tools such as ninja perform escaping on the arguments passed to grit
and that makes it difficult to pass \n at the end of the rc_header_format.

This CL makes the new line at the end implicit by adding automatically.

BUG=338759

Landing for aurimas@chromium.org, patch from issue 183513005.

Review URL: https://codereview.chromium.org/176993010

git-svn-id: http://grit-i18n.googlecode.com/svn/trunk@155 7262f16d-afe8-6277-6482-052fa10e57b1