Fix unused_must_use inside Box

After rust-lang/rust#62228, this will be linted against (and causes the test to fail).

tests/run-pass/issue-30530.rs

@@ -21,7 +21,9 @@ pub enum Handler {
 }
 
 fn main() {
-    take(Handler::Default, Box::new(main));
+    #[allow(unused_must_use)] {
+        take(Handler::Default, Box::new(main));
+    }
 }
 
 #[inline(never)]