compiler: Fix struct inheriting interface with varargs method.

From-SVN: r185060

gcc/go/gofrontend/types.cc

@@ -3744,8 +3744,12 @@ Function_type::copy_with_receiver(Type* receiver_type) const
   go_assert(!this->is_method());
   Typed_identifier* receiver = new Typed_identifier("", receiver_type,
 						    this->location_);
-  return Type::make_function_type(receiver, this->parameters_,
-				  this->results_, this->location_);
+  Function_type* ret = Type::make_function_type(receiver, this->parameters_,
+						this->results_,
+						this->location_);
+  if (this->is_varargs_)
+    ret->set_is_varargs();
+  return ret;
 }
 
 // Make a function type.