selcommands: 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: I22e115ef27d99ff539e04536bdc1c44a53aec978
diff --git a/src/selcommands.cpp b/src/selcommands.cpp
index ee4cfff..d4af84f 100644
--- a/src/selcommands.cpp
+++ b/src/selcommands.cpp
@@ -16,6 +16,7 @@
  */
 
 #include <boost/algorithm/string/join.hpp>
+#include <boost/container/flat_map.hpp>
 #include <ipmid/api.hpp>
 #include <nlohmann/json.hpp>
 #include <phosphor-logging/log.hpp>