dhcpd: fix String -> &str type mismatch in dhcp() call

The dhcp() function takes iface: &str but main() was passing
an owned String. Add & to coerce to &str. Fixes E0308
'mismatched types' during canonical build.

dhcpd/src/main.rs

@@ -395,7 +395,7 @@ fn main() {
         std::thread::sleep(std::time::Duration::from_millis(250));
     }
 
-    if let Err(err) = dhcp(iface, verbose) {
+    if let Err(err) = dhcp(&iface, verbose) {
         eprintln!("dhcpd: {err}");
         process::exit(1);
     }