Fix for br932 part 2 - make $fopen accept SV 'string' type.

The is_string_obj function in vpi/sys_priv.c needed to be updated to
recognise the vpiStringVar object type.

vpi/sys_priv.c

@@ -219,6 +219,7 @@ unsigned is_string_obj(vpiHandle obj)
       case vpiPartSelect:
       case vpiReg:
       case vpiTimeVar:
+      case vpiStringVar:
 	rtn = 1;
 	break;
     }