patch 8.0.1704: 'backupskip' default doesn't work for Mac

Problem:    'backupskip' default doesn't work for Mac.
Solution:   Use "/private/tmp". (Rainer Müller, closes #2793)
diff --git a/src/version.c b/src/version.c
index fdaa1dc..8d7049d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -763,6 +763,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1704,
+/**/
     1703,
 /**/
     1702,