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.

src/mutex.rs

@@ -536,6 +536,7 @@ impl<T: ?Sized, B: Borrow<Mutex<T>>> Drop for AcquireSlow<B, T> {
 }
 
 /// A guard that releases the mutex when dropped.
+#[clippy::has_significant_drop]
 pub struct MutexGuard<'a, T: ?Sized>(&'a Mutex<T>);
 
 unsafe impl<T: Send + ?Sized> Send for MutexGuard<'_, T> {}