Resolve fallthrough warning in mingw build

c++/src/kj/filesystem-disk-win32.c++

@@ -873,6 +873,7 @@ public:
             // Retry, but make sure we don't try to create the parent again.
             return tryReplaceNode(path, mode - WriteMode::CREATE_PARENT, kj::mv(tryCreate));
           }
+          // fallthrough
         default:
           KJ_FAIL_WIN32("create(path)", error, path) { return false; }
       } else {