uapi: stop including linux/sysctl.h in uapi/linux/netfilter.h

linux/netfilter.h is the last uapi header file that includes
linux/sysctl.h but it does not depend on definitions provided
by this essentially dead header file.

Suggested-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>