netfilter.h needs rcuupdate.h for RCU locking functions

This was exposed by Al's recent header file dependency reduction
patches..

Cc: Al Viro <viro@ftp.linux.org.uk>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>