patch 8.2.0577: not all modifiers supported for :options

Problem:    Not all modifiers supported for :options.
Solution:   Use all cmdmod.split flags. (closes #4401)
diff --git a/src/proto/usercmd.pro b/src/proto/usercmd.pro
index a7d0ccc..d7a2a6a 100644
--- a/src/proto/usercmd.pro
+++ b/src/proto/usercmd.pro
@@ -14,5 +14,6 @@
 void ex_comclear(exarg_T *eap);
 void uc_clear(garray_T *gap);
 void ex_delcommand(exarg_T *eap);
+size_t add_win_cmd_modifers(char_u *buf, int *multi_mods);
 void do_ucmd(exarg_T *eap);
 /* vim: set ft=c : */