patch 8.1.0255: backup test fails when using shadow directory

Problem:    Backup test fails when using shadow directory.
Solution:   Remove check for "src".
diff --git a/src/version.c b/src/version.c
index 099ea1f..76d47dd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    255,
+/**/
     254,
 /**/
     253,