hardirq.h: needs sched.h if using BKL

This really isn't the right thing to do, and strictly speaking we should
have the BKL depth count in the thread info right next to the preempt
count.  The two really do go together.

However, since that would involve a patch to all architectures, and the
BKL is finally going away, it's simply not worth the effort to do the
RightThing(tm).  Just re-instate the <linux/sched.h> include that we
used to get accidentally from the smp_lock.h one.

This is all fallout from the same old "BKL: remove extraneous #include
<smp_lock.h>" commit.

Reported-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>