Add missing include for boost::flat_map

A recent update to sdbusplus exposed a missing include. This adds the
include to fix the build.

Signed-off-by: Jason M. Bills <jason.m.bills@intel.com>
Change-Id: I4ff1a69db5f3287a8259dad082e464171b5080b6
diff --git a/service/inc/pfr_mgr.hpp b/service/inc/pfr_mgr.hpp
index d773e4f..8dff6e1 100644
--- a/service/inc/pfr_mgr.hpp
+++ b/service/inc/pfr_mgr.hpp
@@ -19,6 +19,7 @@
 #include "pfr.hpp"
 
 #include <boost/asio.hpp>
+#include <boost/container/flat_map.hpp>
 #include <phosphor-logging/lg2.hpp>
 #include <phosphor-logging/log.hpp>
 #include <sdbusplus/asio/object_server.hpp>