fix socket feature check

it requires you to have at least one socket type enabled but the feature
`socket-dhcp` does not exist because it's name is `socket-dhcpv4`. So if
that's only socket type you want enabled you weren't able to do that due
to this broken check.

src/lib.rs

@@ -104,7 +104,7 @@ compile_error!("You must enable at least one of the following features: proto-ip
         feature = "socket-udp",
         feature = "socket-tcp",
         feature = "socket-icmp",
-        feature = "socket-dhcp",
+        feature = "socket-dhcpv4",
         feature = "socket-dns",
     ))
 ))]