Workaround for when MSVC doesn't have wchar_t as a built-in type

Fixed a problem that TagLib 1.9 doesn't compile with MSVC when ```-Zc:wchar_t``` option is disabled or it doesn't have wchar_t as a built-in type. (reported by @wifanlith in the mailing list)

I'm sorry to cause you trouble @lalinsky, but I hope you adopt the bug fixes #277 and #294 as well.