include/linux/kernel.h: fix a headers_check warning

Fix the warning:

  usr/include/linux/kernel.h:65: userspace cannot reference function or variable defined in the kernel

As Michal noted, BUILD_BUG_ON stuffs should be moved
under #ifdef __KERNEL__.

Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>
Cc: Michal Marek <mmarek@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>