Mark MutexGuard with #[clippy::has_significant_drop]

`#[clippy::has_significant_drop]` tells that a structure should be considered when evaluating some lints. Examples of such behavior are the existent `clippy::significant_drop_in_scrutinee` and in the soon-to-be-finished rust-lang/rust-clippy#9399.

tokio/src/sync/mutex.rs

@@ -141,6 +141,7 @@ pub struct Mutex<T: ?Sized> {
 ///
 /// The lock is automatically released whenever the guard is dropped, at which
 /// point `lock` will succeed yet again.
+#[clippy::has_significant_drop]
 #[must_use = "if unused the Mutex will immediately unlock"]
 pub struct MutexGuard<'a, T: ?Sized> {
     #[cfg(all(tokio_unstable, feature = "tracing"))]