patch 8.2.3753: Vim9: function unreferenced while called is never deleted

Problem:    Vim9: function unreferenced while called is never deleted.
Solution:   Delete a function when no longer referenced.
diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index b5ea9b6..8514448 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -12,6 +12,7 @@
 ufunc_T *find_func(char_u *name, int is_global, cctx_T *cctx);
 int func_is_global(ufunc_T *ufunc);
 int func_name_refcount(char_u *name);
+void func_clear_free(ufunc_T *fp, int force);
 int copy_func(char_u *lambda, char_u *global, ectx_T *ectx);
 int funcdepth_increment(void);
 void funcdepth_decrement(void);