baum.o already receives the sdl cflags in its per object variable, do
the same for brlapi libs to avoid cluttering libs_softmmu.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170907084700.952-1-famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>