Include string when not using dstringt as irep_idt representation

Fixes: #1837
together with all previous commits in this series.

Author: tautschnig

src/util/irep_ids.h

@@ -16,6 +16,8 @@ Author: Reuben Thomas, [email protected]
 
 #ifdef USE_DSTRING
 #include "dstring.h"
+#else
+#include <string>
 #endif
 
 /// \file The irep_ids are generated using a technique called