audio: use try-sdl for openbsd

Fixes the openbsd build failure with SDL disabled.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 20190124112055.547-4-kraxel@redhat.com

configure

@@ -829,7 +829,7 @@ NetBSD)
 OpenBSD)
   bsd="yes"
   make="${MAKE-gmake}"
-  audio_drv_list="sdl"
+  audio_drv_list="try-sdl"
   audio_possible_drivers="sdl"
   HOST_VARIANT_DIR="openbsd"
   supported_os="yes"