Fix build of nu-protocol without plugin feature enabled (#12323)

# Description

I broke this, I think in #12279, because I forgot a `#[cfg(plugin)]`

crates/nu-protocol/src/example.rs

@@ -20,6 +20,7 @@ pub struct PluginExample {
     pub result: Option<Value>,
 }
 
+#[cfg(feature = "plugin")]
 impl From<Example<'_>> for PluginExample {
     fn from(value: Example) -> Self {
         PluginExample {