fix: feature flag Datagram::into_data

`Datagram::into_data` is used in `neqo_common::udp` only. `neqo_common::udp` is
behind the `udp` feature. The feature is not part of the crate's default
features. Thus a plain `cargo check` rightly complains with:

```
warning: method `into_data` is never used
  --> neqo-common/src/datagram.rs:58:19
   |
20 | impl Datagram {
   | ------------- method in this implementation
...
58 |     pub(crate) fn into_data(self) -> Vec<u8> {
   |                   ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default
```

This commit hides `into_data` behind the `udp` feature as well.

neqo-common/src/datagram.rs

@@ -54,6 +54,7 @@ impl Datagram {
         self.ttl
     }
 
+    #[cfg(feature = "udp")]
     #[must_use]
     pub(crate) fn into_data(self) -> Vec<u8> {
         self.d