headers_check: Fix warning text

Fix the warning text too, per Randy.

Cc: Alexander Shishkin <virtuoso@slind.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: WANG Cong <amwang@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>