kobject: Add missing format attribute specifications

Several functions in <linux/kobject.h> accept printf-style arguments.
Some of these functions have been annotated with a format attribute
declaration while others have not been annotated. Add a format attribute
specification where it is missing.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>