Pacify compilation -Wincompatible-pointer-types warnings

Pacify compilation -Wincompatible-pointer-types warnings

* src/w32proc.c (Fw32_get_codepage_charset): Avoid compilation
warning.
(CompareStringW_Proc): New typedef.
(w32_compare_strings): Use it, to pacify compiler warnings under
"-Wincompatible-pointer-types".
* src/w32fns.c (GetDiskFreeSpaceExW_Proc)
(GetDiskFreeSpaceExA_Proc): New typedefs.
(Ffile_system_info): Use them, to pacify compiler warnings under
"-Wincompatible-pointer-types".

Copyright-paperwork-exempt: yes