vt: don't export vt_kmsg_redirect() to userspace

Fix following warning in linux-next by guarding the function definition
(both the "extern" and the inline) with #ifdef __KERNEL__.

usr/include/linux/vt.h:89: userspace cannot call function or variable defined in
the kernel

Introduced by commit 5ada918b ("vt:
introduce and use vt_kmsg_redirect() function").

Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>