patch 9.0.0063: too many type casts for dict_get functions

Problem:    Too many type casts for dict_get functions.
Solution:   Change the key argument from "char_u *" to "char *".
diff --git a/src/filepath.c b/src/filepath.c
index f1ae18e..0867a51 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -1619,7 +1619,7 @@
     }
 
     if (dict_has_key(tv->vval.v_dict, "sort"))
-	compare = dict_get_string(tv->vval.v_dict, (char_u *)"sort", FALSE);
+	compare = dict_get_string(tv->vval.v_dict, "sort", FALSE);
     else
     {
 	semsg(_(e_dictionary_key_str_required), "sort");