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: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/isdn/gigaset/gigaset.h

@@ -20,6 +20,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/kernel.h>
+#include <linux/sched.h>
 #include <linux/compiler.h>
 #include <linux/types.h>
 #include <linux/ctype.h>