osdep: Fix mingw compilation regarding stdio formats

osdep: Fix mingw compilation regarding stdio formats

I encountered the following compilation error on mingw:

/mnt/d/qemu/include/qemu/osdep.h:97:9: error: '__USE_MINGW_ANSI_STDIO' macro redefined [-Werror,-Wmacro-redefined]
 #define __USE_MINGW_ANSI_STDIO 1
        ^
/mnt/d/llvm-mingw/aarch64-w64-mingw32/include/_mingw.h:433:9: note: previous definition is here
 #define __USE_MINGW_ANSI_STDIO 0      /* was not defined so it should be 0 */

It turns out that __USE_MINGW_ANSI_STDIO must be set before any
system headers are included, not just before stdio.h.

Signed-off-by: Cao Jiaxi <driver1998@foxmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Message-id: 20190503003719.10233-1-driver1998@foxmail.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>