patch 9.0.1593: MS-Windows: assert error when compiled with debug mode

Problem:    MS-Windows: assert error when compiled with debug mode.
Solution:   Adjust arguments to setvbuf(). (Ken Takata, closes #12467)
diff --git a/src/proto/main.pro b/src/proto/main.pro
index 307b606..496fe66 100644
--- a/src/proto/main.pro
+++ b/src/proto/main.pro
@@ -3,6 +3,7 @@
 void common_init(mparm_T *paramp);
 int is_not_a_term(void);
 int is_not_a_term_or_gui(void);
+void free_vbuf(void);
 char_u *get_gui_dialog_file(void);
 int op_pending(void);
 void may_trigger_safestate(int safe);