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/ex_getln.c b/src/ex_getln.c
index 894756d..74d7937 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -4879,6 +4879,7 @@
 	{
 	    {EXPAND_COMMANDS, get_command_name, FALSE, TRUE},
 	    {EXPAND_BEHAVE, get_behave_arg, TRUE, TRUE},
+	    {EXPAND_MAPCLEAR, get_mapclear_arg, TRUE, TRUE},
 	    {EXPAND_MESSAGES, get_messages_arg, TRUE, TRUE},
 #ifdef FEAT_CMDHIST
 	    {EXPAND_HISTORY, get_history_arg, TRUE, TRUE},