checkpatch: make extern in .h prototypes quieter

The use of extern in .h files is a bit contentious.

Make the warning be emitted only when --strict is used on the command
line.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>