Add borgmatic config to .gitignore (#4061)

Signed-off-by: Felix Kaechele <felix@kaechele.ca>

.gitignore

@@ -7,6 +7,7 @@
 data/assets/ejabberd/sqlite/sqlite.db
 data/assets/ssl-example/*
 data/assets/ssl/*
+data/conf/borgmatic/
 data/conf/clamav/whitelist.ign2
 data/conf/dovecot/acl_anyone
 data/conf/dovecot/dovecot-master.passwd