ui: avoid warnings about directdb on Alpine / musl libc

On Alpine, SDL is built with directfb support and this triggers warnings
during QEMU build

In file included from /usr/include/directfb/direct/thread.h:38,
                 from /usr/include/directfb/direct/debug.h:43,
                 from /usr/include/directfb/direct/interface.h:36,
                 from /usr/include/directfb/directfb.h:49,
                 from /usr/include/SDL2/SDL_syswm.h:80,
                 from /builds/berrange/qemu/include/ui/sdl2.h:8,
                 from ../ui/sdl2-gl.c:31:
/usr/include/directfb/direct/os/waitqueue.h:41:25: error: redundant redeclaration of 'direct_waitqueue_init' [-Werror=redundant-decls]
   41 | DirectResult DIRECT_API direct_waitqueue_init        ( DirectWaitQueue *queue );
      |                         ^~~~~~~~~~~~~~~~~~~~~

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211215141949.3512719-5-berrange@redhat.com>
Message-Id: <20220105135009.1584676-5-alex.bennee@linaro.org>