mpls: fix af_mpls dependencies for real

Randy reported that selecting MPLS_ROUTING without PROC_FS breaks
the build, because since commit c1a9d659 ("mpls: fix af_mpls
dependencies"), MPLS_ROUTING selects PROC_SYSCTL, but Kconfig's select
doesn't recursively handle dependencies.
Change the select into a dependency.

Fixes: c1a9d659 ("mpls: fix af_mpls dependencies")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Matteo Croce <mcroce@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>