patch 8.0.0878: no completion for :mapclear

Problem:    No completion for :mapclear.
Solution:   Add completion (Nobuhiro Takasaki et al. closes #1943)
diff --git a/src/proto/ex_docmd.pro b/src/proto/ex_docmd.pro
index 11a3f71..1a931e5 100644
--- a/src/proto/ex_docmd.pro
+++ b/src/proto/ex_docmd.pro
@@ -62,4 +62,5 @@
 void dialog_msg(char_u *buff, char *format, char_u *fname);
 char_u *get_behave_arg(expand_T *xp, int idx);
 char_u *get_messages_arg(expand_T *xp, int idx);
+char_u *get_mapclear_arg(expand_T *xp, int idx);
 /* vim: set ft=c : */