sync: Add #[must_use] to the Mutex guard.

This helps people remember to save the return value to keep the mutex
locked as appropriate.

src/libsync/sync/mutex.rs

@@ -143,6 +143,7 @@ pub struct StaticMutex {
 
 /// An RAII implementation of a "scoped lock" of a mutex. When this structure is
 /// dropped (falls out of scope), the lock will be unlocked.
+#[must_use]
 pub struct Guard<'a> {
     priv lock: &'a mut StaticMutex,
 }