kbuild/headers_check: refine extern check

'extern' checking information is not clear, refine it.
Plus, fix a comment.

Signed-off-by: WANG Cong <amwang@redhat.com>
[sam: redid the extern error message]
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>