Mpls and LinkBridgeVlan introduction break FreeBSD builds

[ydirson]

Full output in https://gitlab.com/xen-project/xen-guest-agent/-/jobs/13277676163

LinkBridgeVlan usage seems to lack some conditionals:

error[E0432]: unresolved import `crate::link::LinkBridgeVlan`
  --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rtnetlink-0.20.0/src/lib.rs:53:9
   |
53 |         LinkBridgeVlan, LinkDelPropRequest, LinkDelRequest, LinkDummy,
   |         ^^^^^^^^^^^^^^
   |         |
   |         no `LinkBridgeVlan` in `link`
   |         help: a similar name exists in the module: `LinkBridge`
   |
note: found an item that was configured out
  --> /root/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rtnetlink-0.20.0/src/link/mod.rs:63:28

I see Mpls is already excluded on Android, it seems to be needed to exclude it on FreeBSD as well.