Fix missing implicit include.
Patch by Jan-Matthias Braun
See also: https://bugs.gentoo.org/show_bug.cgi?id=598340

--- a/src/mongo/client/examples/connect.cpp
+++ b/src/mongo/client/examples/connect.cpp
@@ -29,6 +29,8 @@
 #include "mongo/client/dbclient.h"
 #include "mongo/stdx/functional.h"
 
+#include <boost/core/ref.hpp>
+
 namespace {
 
 class OstreamAppender : public mongo::logger::MessageLogDomain::EventAppender {