patch 9.0.0350: :echowindow does not work in a compiled function

Problem:    :echowindow does not work in a compiled function.
Solution:   Handle the expression at compile time.
diff --git a/src/channel.c b/src/channel.c
index 8977686..80fe4c4 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -2731,12 +2731,8 @@
     }
     else if (STRCMP(cmd, "redraw") == 0)
     {
-	exarg_T ea;
-
 	ch_log(channel, "redraw");
-	CLEAR_FIELD(ea);
-	ea.forceit = *arg != NUL;
-	ex_redraw(&ea);
+	redraw_cmd(*arg != NUL);
 	showruler(FALSE);
 	setcursor();
 	out_flush_cursor(TRUE, FALSE);