Fix ISDN/Gigaset build failure

Commit b91ecb00 ("gigaset: include cleanup cleanup") removed an implicit
sched.h inclusion that came in via slab.h, and caused various compile
problems as a result.

This should fix it.

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>