patch 9.0.0288: when 'cmdheight' is zero some messages are not displayed

Problem:    When 'cmdheight' is zero some messages are not displayed.
Solution:   Use a popup notification window.
diff --git a/src/proto/message.pro b/src/proto/message.pro
index 3f8a8fe..bea3016 100644
--- a/src/proto/message.pro
+++ b/src/proto/message.pro
@@ -23,6 +23,7 @@
 void wait_return(int redraw);
 void set_keep_msg(char_u *s, int attr);
 void set_keep_msg_from_hist(void);
+int use_message_window(void);
 void msg_start(void);
 void msg_starthere(void);
 void msg_putchar(int c);