bug.h: need linux/kernel.h for TAINT_WARN.

asm-generic/bug.h uses taint flags that are only defined in
linux/kernel.h, resulting in build failures on platforms that
don't include linux/kernel.h some other way:

        arch/sh/include/asm/thread_info.h:172:2: error: 'TAINT_WARN' undeclared (first use in this function)

Caused by commit edd63a27 ("set_restore_sigmask() is never called
without SIGPENDING (and never should be)").

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>