basic_event_log_backend constructor requires keywords::message_file

[GregDomjan]

A bit of a nuisance as I'm not familiar enough with these argument list constructors.

MSVC reports
sinks/event_log_backend.hpp(628): error C2679: binary '[' : no operator found which takes a right-hand operand of type 'const boost::parameter::keyword<boost::log::v2s_mt_nt5::keywords::tag::message_file>' (or there is no acceptable conversion)

https://github.com/boostorg/log/blob/master/include/boost/log/sinks/event_log_backend.hpp#L625-633

Even when keywords::registration = sinks::event_log::never and keywords::message_file and keywords::log_name are unused, keywords::message_file must be provided - perhaps as there is no default for it?

// Create an event log sink
boost::shared_ptr< sinks::wevent_log_backend > backend(
    new sinks::wevent_log_backend((
    keywords::message_file = L"",// "%SystemDir%\\event_log_messages.dll",  not initialising registry, don't need to provide the message dll name - if this isn't provided there is a compiler error here in MSVC
    //keywords::log_name = L"",// "Application",  not initializing registry, don't need to provide the named log 
    keywords::log_source = L"My Source",
    keywords::registration = sinks::event_log::never
    ))
    );

[Lastique]

Changed in cb68c02. Thanks.