Ignore warnings on missing virtual destructor

The boost::system::error_category has a non virtual destructor but
virtual functions. There's probably a good reason for that and not
much to do about it so silence that reasonable warning from GCC when
inheriting from boost::system::error_category

test/test_stream/impl/error.ipp

@@ -12,6 +12,11 @@
 
 #include "error.hpp"
 
+#ifdef __MINGW32__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#endif
+
 namespace wintls {
 namespace test {
 
@@ -45,6 +50,10 @@ public:
  }
 };
 
+#ifdef __MINGW32__
+#pragma GCC diagnostic pop
+#endif
+
 inline
 error_code
 make_error_code(wintls::test::error e) noexcept