lavc/libaomenc: Mark a potentially unused variable as av_unused.

Fixes a warning:
libavcodec/libaomenc.c:816:9: warning: unused variable ‘pict_type’

libavcodec/libaomenc.c

@@ -813,7 +813,7 @@ static int storeframe(AVCodecContext *avctx, struct FrameListData *cx_frame,
                       AVPacket *pkt)
 {
     AOMContext *ctx = avctx->priv_data;
-    int pict_type;
+    int av_unused pict_type;
     int ret = ff_alloc_packet2(avctx, pkt, cx_frame->sz, 0);
     if (ret < 0) {
         av_log(avctx, AV_LOG_ERROR,