patch 9.0.0551: mode message is delayed when :echowin was used

Problem:    Mode message is delayed when :echowin was used. (Maxim Kim)
Solution:   Save and restore msg_didout in :echowin. (closes #11193)
diff --git a/src/version.c b/src/version.c
index 5d6951c..c3b6c12 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    551,
+/**/
     550,
 /**/
     549,