headers/uninline: Uninline single-use function: kobject_has_children()

This was the only usage of <linux/kref_api.h> in <linux/kobject_api.h>,
so we'll able to decouple the two after this change.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>