lavc/libaomdec: fix build with 1.0.0

aom_codec_frame_flags_t in libaom 1.0.0 is defined in aom_encoder.h,
whereas for newer versions it was moved to aom_codec.h

libavcodec/libaomdec.c

@@ -161,7 +161,6 @@ static int aom_decode(AVCodecContext *avctx, void *data, int *got_frame,
     AVFrame *picture      = data;
     const void *iter      = NULL;
     struct aom_image *img;
-    aom_codec_frame_flags_t av_unused flags;
     int ret;
 
     if (aom_codec_decode(&ctx->decoder, avpkt->data, avpkt->size, NULL) !=
@@ -200,6 +199,8 @@ static int aom_decode(AVCodecContext *avctx, void *data, int *got_frame,
             return ret;
 
 #ifdef AOM_CTRL_AOMD_GET_FRAME_FLAGS
+        {
+            aom_codec_frame_flags_t flags;
             ret = aom_codec_control(&ctx->decoder, AOMD_GET_FRAME_FLAGS, &flags);
             if (ret == AOM_CODEC_OK) {
                 picture->key_frame = !!(flags & AOM_FRAME_IS_KEY);
@@ -210,6 +211,7 @@ static int aom_decode(AVCodecContext *avctx, void *data, int *got_frame,
                 else
                     picture->pict_type = AV_PICTURE_TYPE_P;
             }
+        }
 #endif
 
         av_reduce(&picture->sample_aspect_ratio.num,