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: I1b6868a48bb55745b7d7ac6eed61e3301afd160e
diff --git a/src/host_error_monitor.cpp b/src/host_error_monitor.cpp
index 22754be..0f34535 100644
--- a/src/host_error_monitor.cpp
+++ b/src/host_error_monitor.cpp
@@ -14,6 +14,7 @@
 // limitations under the License.
 */
 #include <boost/asio/io_service.hpp>
+#include <boost/container/flat_map.hpp>
 #include <error_monitors.hpp>
 #include <host_error_monitor.hpp>
 #include <sdbusplus/asio/object_server.hpp>