Mark the Sine example struct pub

For standalone testing. The `Cargo.toml` and `main.rs` changes aren't
committed since these plugins don't really need standalones, but it's
still nice for debugging.

plugins/examples/sine/src/lib.rs

@@ -4,7 +4,7 @@ use std::sync::Arc;
 
 /// A test tone generator that can either generate a sine wave based on the plugin's parameters or
 /// based on the current MIDI input.
-struct Sine {
+pub struct Sine {
     params: Arc<SineParams>,
     sample_rate: f32,