patch 8.1.1519: 'backupskip' may contain duplicates

Problem:    'backupskip' may contain duplicates.
Solution:   Add the P_NODUP flag. (Tom Ryder)
diff --git a/src/version.c b/src/version.c
index 7432f70..5c41f4f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1519,
+/**/
     1518,
 /**/
     1517,