Add gitignore for bak files in nginx conf dir

.gitignore

@@ -19,5 +19,6 @@ data/conf/rspamd/override.d/*
 !data/conf/nginx/site.conf
 data/conf/nginx/*.conf
 data/conf/nginx/*.custom
+data/conf/nginx/*.bak
 data/conf/dovecot/extra.conf
 data/conf/rspamd/custom/*