patch 8.1.0264: backup tests fail when CWD is in /tmp

Problem:    Backup tests fail when CWD is in /tmp.
Solution:   Make 'backupskip' empty. (Christian Brabandt, closes #3301)
diff --git a/src/version.c b/src/version.c
index 6543e91..2d04731 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    264,
+/**/
     263,
 /**/
     262,