agent: Use vec![] macro rather than init-then-push

We have one place where we create an empty vector then immediately push
something into it.  We can do this in one step using the vec![] macro,
which stops clippy complaining.

fixes #1611

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/agent/src/netlink.rs

@@ -542,12 +542,10 @@ impl Handle {
                 ntype: NDA_UNSPEC as u8,
             },
             nlas: {
-                let mut nlas = vec![];
-
-                nlas.push(Nla::Destination(match ip {
+                let mut nlas = vec![Nla::Destination(match ip {
                     IpAddr::V4(v4) => v4.octets().to_vec(),
                     IpAddr::V6(v6) => v6.octets().to_vec(),
-                }));
+                })];
 
                 if !neigh.lladdr.is_empty() {
                     nlas.push(Nla::LinkLocalAddress(