clar: avoid compile error with mingw-w64

clar: avoid compile error with mingw-w64

When using mingw-w64 to compile the code, and using `_stat()`, it is
necessary to use `struct _stat`, too, and not `struct stat` (as the
latter is incompatible with the "dashed" version because it is limited
to 32-bit time types for backwards compatibility).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>