X25: Move SYSCTL ifdefs into header

Moves the CONFIG_SYSCTL ifdefs in x25_init into header.

Signed-off-by: Andrew Hendry <andrew.hendry@gmail.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>