Add missing include for std::map

The file relies on irep.h providing std::map, which it does not do if
setting NAMED_SUB_IS_FORWARD_LIST.

Author: tautschnig

src/util/irep_hash_container.h

@@ -12,6 +12,7 @@ Author: Daniel Kroening, kroening@kroening.com
 #ifndef CPROVER_UTIL_IREP_HASH_CONTAINER_H
 #define CPROVER_UTIL_IREP_HASH_CONTAINER_H
 
+#include <map>
 #include <vector>
 
 #include "irep.h"