mpls: fix af_mpls dependencies

MPLS routing code relies on sysctl to work, so let it select PROC_SYSCTL.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>