slab: Fix comment on #endif

This #endif in slab.h is described as closing the inner block while it's for
the big CONFIG_NUMA one. That makes reading the code a bit harder.

This trivial patch fixes the comment.

Signed-off-by: Pascal Terjan <pterjan@mandriva.com>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>