mingw32: Enable C99/POSIX format strings

mingw32: Enable C99/POSIX format strings

Starting with mingw32-runtime 3.15, C99/POSIX
format strings (%zu, %lld, ...) are supported
by defining __USE_MINGW_ANSI_STDIO=1.

As QEMU uses such format strings, unconditionally
define this macro. It won't hurt on older revisions
of mingw32-runtime.

Tested with manually installed mingw32-runtime 3.15
on debian (cross compiled + wine).

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>