patch 8.1.1551: warning for shadowing popup_dragwin

Problem:    Warning for shadowing popup_dragwin. (Dominique Pelle)
Solution:   Add missing change.
diff --git a/src/version.c b/src/version.c
index 96670c6..b213bda 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1551,
+/**/
     1550,
 /**/
     1549,