Motivation / Problem

OTTD2FS() now returns std::wstring on windows and MinGW fails to compile because there's no std::ofstream(std::wstring) constructor (but MSVC has one).
For other platforms OTTD2FS() returns std::string and std::ofstream(std::string) constructor exists.

Description

Use std::ofstream(wchar_t *) constructor instead for windows, other platforms will use std::ofstream(char *).

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker