Add missing include for boost::next()

https://bugs.gentoo.org/671422
--- a/src/mongo/client/command_writer.cpp
+++ b/src/mongo/client/command_writer.cpp
@@ -22,4 +22,6 @@
 #include "mongo/db/namespace_string.h"
 
+#include <boost/next_prior.hpp>
+
 namespace mongo {
 
--- a/src/mongo/client/wire_protocol_writer.cpp
+++ b/src/mongo/client/wire_protocol_writer.cpp
@@ -22,4 +22,6 @@
 #include "mongo/db/namespace_string.h"
 
+#include <boost/next_prior.hpp>
+
 namespace mongo {
 
