Include config.hpp before checking BOOST_LOG_WITHOUT_SYSLOG.

Boost.Log's config.hpp may define BOOST_LOG_WITHOUT_SYSLOG if no native
syslog API or Boost.ASIO is available, so we need to include the header
before checking the macro.

Closes https://github.com/boostorg/log/pull/123.

src/syslog_backend.cpp

@@ -13,9 +13,10 @@
  *         at http://www.boost.org/doc/libs/release/libs/log/doc/html/index.html.
  */
 
+#include <boost/log/detail/config.hpp>
+
 #ifndef BOOST_LOG_WITHOUT_SYSLOG
 
-#include <boost/log/detail/config.hpp>
 #include <ctime>
 #include <algorithm>
 #include <stdexcept>