linux/futex.h: place kernel types behind __KERNEL__

The forward decls for some kernel types are only needed by the code behind
__KERNEL__, so don't bleed these types to userspace.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>