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/fileio.c b/src/fileio.c
index 2c6c338..4287e8f 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -4746,8 +4746,8 @@
 {
     char_u  *name1, *name2;
 
-    name1 = dict_get_string(*(dict_T**)p1, (char_u*)"name", FALSE);
-    name2 = dict_get_string(*(dict_T**)p2, (char_u*)"name", FALSE);
+    name1 = dict_get_string(*(dict_T**)p1, "name", FALSE);
+    name2 = dict_get_string(*(dict_T**)p2, "name", FALSE);
     if (readdirex_sort == READDIR_SORT_BYTE)
 	return STRCMP(name1, name2);
     else if (readdirex_sort == READDIR_SORT_IC)