patch 8.1.1604: popup window scroll test is flaky

Problem:    Popup window scroll test is flaky.
Solution:   Add a delay between scroll events.
diff --git a/src/version.c b/src/version.c
index 26a9bf5..9c29b46 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1604,
+/**/
     1603,
 /**/
     1602,