patch 8.1.2020: it is not easy to change the window layout

Problem:    It is not easy to change the window layout.
Solution:   Add win_splitmove(). (Andy Massimino, closes #4561)
diff --git a/src/version.c b/src/version.c
index afbf0bc..44b6a51 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2020,
+/**/
     2019,
 /**/
     2018,