Add xtrabackup to fs_excludes list

Change-Id: I8f5ec208bb0132c009eaf37ef88ef4953818e592
Prod-Related: PROD-20383
diff --git a/mysql/meta/backupninja.yml b/mysql/meta/backupninja.yml
index 80c192c..36daf17 100644
--- a/mysql/meta/backupninja.yml
+++ b/mysql/meta/backupninja.yml
@@ -2,4 +2,5 @@
   mysql:
     fs_includes:
     - /var/backups/mysql
-    fs_excludes: []
+    fs_excludes:
+    - xtrabackup