vd_lavc: fix build with FFmpeg 3.2

Sigh...

The functionality is not actually needed for vdpau, but if the vdpau
hwaccel is present, the FFmpeg version is new enough that it includes
the field.

video/decode/vd_lavc.c

@@ -591,8 +591,13 @@ static void init_avctx(struct dec_video *vd, const char *decoder,
             ctx->hwdec_dev = hwdec_create_dev(vd, ctx->hwdec, false);
             if (!ctx->hwdec_dev)
                 goto error;
-            if (!ctx->hwdec->set_hwframes)
+            if (!ctx->hwdec->set_hwframes) {
+#if HAVE_VDPAU_HWACCEL
                 avctx->hw_device_ctx = av_buffer_ref(ctx->hwdec_dev->av_device_ref);
+#else
+                goto error;
+#endif
+            }
         }
         ctx->max_delay_queue = ctx->hwdec->delay_queue;
         ctx->hw_probing = true;