Fix parsing of "ldap_dn_filter" option

Without this fix, the following configuration snippet ([suggested][1] in the guide) ...

    ldap_dn_filter:
      "(&(name=%s)(owner=%D)(user=%u@%d))": ["sn"]

... leads to the following error message:

    ** Option: {ldap_dn_filter,global}
    ** Invalid value: [{"(&(name=%s)(owner=%D)(user=%u@%d))",["sn"]}]
    ** Using as fallback: {undefined,[]}

[1]: http://www.process-one.net/docs/ejabberd/guide_en.html#ldapauth