For mingw-w64, pid_t is _pid_t which is __int64,
so this platform needs its own definition of FMT_pid.

Reviewed-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>