browser.c: Fix "Toggle dirs before files when sorting" functionality

This has apparently been broken since version 1.7 (v1.6-7-g5db9c2a to be
precise), yet NOBODY TOLD ME!? :-(

The OSS community has left me in despair!

src/browser.c

@@ -350,7 +350,7 @@ int browse_key(int ch) {
       info_show = 0;
       break;
     case 't':
-      dirlist_set_sort(DL_NOCHANGE, DL_NOCHANGE, dirlist_sort_df);
+      dirlist_set_sort(DL_NOCHANGE, DL_NOCHANGE, !dirlist_sort_df);
       info_show = 0;
       break;
     case 'a':