openvswitch: partially restore kmod-mpls dependency

openvswitch: partially restore kmod-mpls dependency

Enabling OPENVSWITCH in the kernel config selects MPLS. This exposes the
MPLS_ROUTING symbol, which is missing if kmod-mpls is not enabled. On
kernel 5.4 this problem doesn't show up, as the Open vSwitch package
uses the in-tree kernel modules rather than the upstream ones.

Restore the kmod-mpls dependency when using the upstream kernel modules
to fix build.

Reported-by: Matthew Hagan <mnhagan88@gmail.com>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>