QStringConverterBase points to qstringconverter.h, not qstringconverter_base.h

The header QStringConverterBase points to qstringconverter.h and not qstringconverter_base.h as one would expect.

I'm not sure if this is public API because I couldn't find documentation for QStringConverterBase. If not, it would be nice to add a IWYU pragma: export to qstringconverter.h when including{{ qstringconverter_base.h}} so clang-tidy doesn't suggest including the latter.