srvcfg_manager: add include for flat_map

We were getting this include for free from sdbusplus, but it has
gone away in a recent commit.  Add the appropriate include.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I4488f27923e16eb8110d888e2fbdaf72f6db2da7
diff --git a/inc/srvcfg_manager.hpp b/inc/srvcfg_manager.hpp
index 2aa8f47..b0e345c 100644
--- a/inc/srvcfg_manager.hpp
+++ b/inc/srvcfg_manager.hpp
@@ -16,6 +16,7 @@
 #pragma once
 #include "utils.hpp"
 
+#include <boost/container/flat_map.hpp>
 #include <sdbusplus/timer.hpp>
 
 namespace phosphor