util: use gnu_printf format attribute

dtc uses non-portable formats. Using gnu_printf attributes (for
warnings) in combination with __USE_MINGW_ANSI_STDIO allows to build
for win32.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191009102025.10179-8-marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>