GLib 2.0 include files use __printf__ for the format attribute
which resolves to native format strings on w32 hosts.

QEMU wants standard format strings instead of native format
strings, so we simply change any declaration with __printf__
to use __gnu_printf__.

This works because all basic printf functions support both
kinds of format strings.

This fixes a compiler warning:

qapi/string-output-visitor.c: In function ‘print_type_int’:
qapi/string-output-visitor.c:34:5: warning: unknown conversion type character ‘l’ in format [-Wformat]
qapi/string-output-visitor.c:34:5: warning: too many arguments for format [-Wformat-extra-args]

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>