writeentity: add missing header utility

The code includes utility indirectly. Utility is required for make_pair().

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I3cdca868a69ca67363f2e3692cbfd511dfde7b5f
diff --git a/scripts/writeentity.mako.cpp b/scripts/writeentity.mako.cpp
index 4cfc82b..110eca9 100644
--- a/scripts/writeentity.mako.cpp
+++ b/scripts/writeentity.mako.cpp
@@ -3,6 +3,8 @@
 // !!! WARNING: This is a GENERATED Code..Please do NOT Edit !!!
 
 #include <ipmid/types.hpp>
+#include <utility>
+
 using namespace ipmi::sensor;
 
 extern const EntityInfoMap entities = {