patch 9.0.1516: cannot use special keys in <Cmd> mapping

Problem:    Cannot use special keys in <Cmd> mapping.
Solution:   Do allow for special keys in <Cmd> and <ScriptCmd> mappings.
            (closes #12326)
diff --git a/src/proto/getchar.pro b/src/proto/getchar.pro
index 66c1e9b..01746cf 100644
--- a/src/proto/getchar.pro
+++ b/src/proto/getchar.pro
@@ -11,6 +11,7 @@
 void restoreRedobuff(save_redo_T *save_redo);
 void AppendToRedobuff(char_u *s);
 void AppendToRedobuffLit(char_u *str, int len);
+void AppendToRedobuffSpec(char_u *s);
 void AppendCharToRedobuff(int c);
 void AppendNumberToRedobuff(long n);
 void stuffReadbuff(char_u *s);