init/do_mounts.c: proper prepare_namespace() prototype

Add a proper protype for prepare_namespace() in include/linux/init.h.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>