MSVC: Fix an "incompatible pointer types" compiler warning

MSVC: Fix an "incompatible pointer types" compiler warning

In particular, the following warning is issued while compiling
compat/msvc.c:

    ...mingw.c(223) : warning C4133: 'function' : incompatible \
types - from '_stati64 *' to '_stat64 *'

which relates to a call of _fstati64() in the mingw_fstat()
function definition.

This is caused by various layers of macro magic and attempts to
avoid macro redefinition compiler warnings. For example, the call
to _fstati64() mentioned above is actually a call to _fstat64(),
and expects a pointer to a struct _stat64 rather than the struct
_stati64 which is passed to mingw_fstat().

The definition of struct _stati64 given in compat/msvc.h had the
same "shape" as the definition of struct _stat64, so the call to
_fstat64() does not actually cause any runtime errors, but the
structure types are indeed incompatible.

In order to avoid the compiler warning, we add declarations for the
mingw_lstat() and mingw_fstat() functions and supporting macros to
msvc.h, suppressing the corresponding declarations in mingw.h, so
that we can use the appropriate structure type (and function) names
from the msvc headers.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>