Include missing <cassert>

We are currently transitively depending on sdbusplus to provide the
definition of assert(). Add the necessary include we should have
requested.

Change-Id: I9e348edc851fc922916942f522748f0bc59d5399
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/bmc_state_manager.cpp b/bmc_state_manager.cpp
index 1e63f26..6651d2f 100644
--- a/bmc_state_manager.cpp
+++ b/bmc_state_manager.cpp
@@ -1,3 +1,4 @@
+#include <cassert>
 #include <phosphor-logging/log.hpp>
 #include <sys/sysinfo.h>
 #include "bmc_state_manager.hpp"