Merge #2470: Remove char_u (5)

Reviewed-by: Scott Prager <splinterofchaos@gmail.com>
Reviewed-by: Michael Reed <m.reed@mykolab.com>
Reviewed-by: Eliseo Martínez <eliseomarmol@gmail.com>

src/nvim/undo.c

@@ -677,7 +677,7 @@ char_u *u_get_undo_file_name(char_u *buf_ffname, int reading)
             if (vim_ispathsep(*p))
               *p = '%';
         }
-        undo_file_name = concat_fnames(dir_name, munged_name, TRUE);
+        undo_file_name = (char_u *)concat_fnames((char *)dir_name, (char *)munged_name, TRUE);
       }
     }