Fix race condition when adding a dependency

src/checker.hpp

@@ -300,7 +300,8 @@ struct CheckerInfo {
 	BlockingMutex global_untyped_mutex;
 	BlockingMutex builtin_mutex;
 
-	// NOT recursive & Only used at the end of `check_proc_body`
+	// NOT recursive & only used at the end of `check_proc_body`
+	// and in `add_dependency`.
 	// This is a possible source of contention but probably not
 	// too much of a problem in practice
 	BlockingMutex deps_mutex;