add missing use statement

Author: ziali088

lib/Net/SAML2/Protocol/LogoutRequest.pm

@@ -3,6 +3,7 @@ use Moose;
 use MooseX::Types::Common::String qw/ NonEmptySimpleStr /;
 use MooseX::Types::URI qw/ Uri /;
 use Net::SAML2::XML::Util qw/ no_comments /;
+use XML::Generator;
 
 with 'Net::SAML2::Role::ProtocolMessage';