checkpatch: __weak is an official attribute

Add __weak as an official attribute.  This tends to be used in a location
where the automated attribute detector misses it.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>