Netlist conversion: do not convert types

This prevents passing SystemVerilog enums to the decision procedure, which
causes an error.

Fixes #623.

Author: kroening

src/trans-netlist/trans_to_netlist.cpp

@@ -207,6 +207,8 @@ void convert_trans_to_netlistt::map_vars(
     else if (symbol.type.id() == ID_module ||
              symbol.type.id() == ID_module_instance)
       return; // ignore modules
+    else if(symbol.is_type)
+      return; // ignore types
     else if (symbol.is_input)
       vartype = var_mapt::vart::vartypet::INPUT;
     else if (symbol.is_state_var)