Fix build when with no socket features

[Dirbaio]

Building like this

cargo test --no-default-features --features std,proto-ipv4

fails with this

error[E0392]: parameter `'a` is never used
--> src/socket/mod.rs:84:17
|
84 | pub enum Socket<'a> {
|                 ^^ unused parameter

This used to work before, was broken by #410 . Rather than adding back a dummy enum variant, this fixes the issue by making the entire socket module optional, only built if at least one socket feature is enabled.