Silence Cygwin warning "'vsprintf_s' redeclared without dllimport..."

Silence Cygwin warning "'vsprintf_s' redeclared without dllimport..."

Silence a compiler warning which used to didn't show up with some
old version of Cygwin. ad9c0fbc in 2012 added this, but it seems
this prototype should have been declared by an appropriate header
that's already been included here, so drop this prototype.
It also sounds what this message means according to Google.

Feel free to drop this if this is really needed for some reason.

  os/windows/posix.c:43:5: warning: 'vsprintf_s' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
   int vsprintf_s(
       ^

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>