patch 8.1.1441: popup window filter not yet implemented

Problem:    Popup window filter not yet implemented.
Solution:   Implement the popup filter.
diff --git a/src/proto/misc2.pro b/src/proto/misc2.pro
index 84ed66a..43921ed 100644
--- a/src/proto/misc2.pro
+++ b/src/proto/misc2.pro
@@ -69,6 +69,7 @@
 int handle_x_keys(int key);
 char_u *get_special_key_name(int c, int modifiers);
 int trans_special(char_u **srcp, char_u *dst, int keycode, int in_string);
+int special_to_buf(int key, int modifiers, int keycode, char_u *dst);
 int find_special_key(char_u **srcp, int *modp, int keycode, int keep_x_key, int in_string);
 int extract_modifiers(int key, int *modp);
 int find_special_key_in_table(int c);