txd: Fix "set but not used" warning

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/txd.c

@@ -47,7 +47,8 @@ static int txd_decode_frame(AVCodecContext *avctx, void *data, int *data_size,
     GetByteContext gb;
     AVFrame *picture = data;
     AVFrame * const p = &s->picture;
-    unsigned int version, w, h, d3d_format, depth, stride, mipmap_count, flags;
+    unsigned int version, w, h, d3d_format, depth, stride, flags;
+    unsigned int av_unused mipmap_count;
     unsigned int y, v;
     uint8_t *ptr;
     uint32_t *pal;