patch 8.1.1770: cannot get the window ID of the popup preview window

Problem:    Cannot get the window ID of the popup preview window.
Solution:   Add popup_getpreview().
diff --git a/src/version.c b/src/version.c
index 2db2ee1..b0fe67f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1770,
+/**/
     1769,
 /**/
     1768,