Fix build on OpenBSD/amd64 7.4 (Clang 16.0.6)

As vendored copy in Telegram's official desktop client, specificially bundled in https://github.com/desktop-app/cmake_helpers
tdesktop 4.14.0 fails to build on bespoke system:
```
../tdesktop-4.14.1-full/cmake/external/glib/cppgir/tools/genutils.hpp:125:26: error: implicit instantiation of undefined template 'std::basic_istringstream<char>'
      std::istringstream iss(patterns);
                         ^
/usr/include/c++/v1/iosfwd:136:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_istringstream;
                               ^
1 error generated.
```

Including `<sstream>` lets tdesktop 4.14.0/4.14.1 and thus cppgir build again.