sched/wait: Include <linux/wait.h> in <linux/swait.h>

kbuild bot reported against an intermediate RT patch that the build
fails with:

>    In file included from include/linux/completion.h:12:0,
>                     from include/linux/rcupdate_wait.h:10,
>                     from kernel/rcu/srcutiny.c:27:
>    kernel/rcu/srcutiny.c: In function 'srcu_drive_gp':
> >> include/linux/swait.h:172:7: error: implicit declaration of function '___wait_is_interruptible'; did you mean '__swait_event_interruptible'?
>       if (___wait_is_interruptible(state) && __int) {  \

That error vanishes a few patches later (in the RT queue) because wait.h
is then pulled in by other means. It does not seem to surface on !RT.
I think that swait should include a header file for a function/macro
(___wait_is_interruptible()) it is using.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/20180504104224.20218-1-bigeasy@linutronix.deSigned-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/swait.h

@@ -5,6 +5,7 @@
 #include <linux/list.h>
 #include <linux/stddef.h>
 #include <linux/spinlock.h>
+#include <linux/wait.h>
 #include <asm/current.h>
 
 /*