headers_check: better search for functions in headers

Some headers don't bother with "extern" in function prototypes, which
results in said prototypes being unnoticed and exported to userland.

This patch slightly improves detection of such cases by checking for C
type names as well in the beginning of a line.

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