Fix MSVC compatibility by using vswprintf instead of snwprintf

[ttytm]

I was running into the issue of not being able to use the library with MSVC.

osdialog.lib(osdialog_win.obj) : error LNK2019: unresolved external symbol snwprintf referenced in function osdialog_file
C:\Users\vboxuser\Dev\main.exe : fatal error LNK1120: 1 unresolved externals

Reason is that snwprintf is not available in MSVC. Instead, MSVC provides _snwprintf. Though to ensure compatibility with both MSVC and MinGW, I changed it to a vswprintf call.

[ttytm]

I see #17 is already there since 4+ years