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>

net/mpls/Kconfig

@@ -26,7 +26,7 @@ config NET_MPLS_GSO
 config MPLS_ROUTING
 	tristate "MPLS: routing support"
 	depends on NET_IP_TUNNEL || NET_IP_TUNNEL=n
-	select PROC_SYSCTL
+	depends on PROC_SYSCTL
 	---help---
 	 Add support for forwarding of mpls packets.