libavcodec/zmbv: use PTRDIFF_SPECIFIER for `src - c->decomp_buf`.

Other bit depths saw this change in ced0d6c14d, but this instance was
presumably missed because of the #ifdef block.

libavcodec/zmbv.c

@@ -303,7 +303,7 @@ static int zmbv_decode_xor_24(ZmbvContext *c)
         prev += stride * c->bh;
     }
     if (src - c->decomp_buf != c->decomp_len)
-        av_log(c->avctx, AV_LOG_ERROR, "Used %i of %i bytes\n",
+        av_log(c->avctx, AV_LOG_ERROR, "Used %"PTRDIFF_SPECIFIER" of %i bytes\n",
                src-c->decomp_buf, c->decomp_len);
     return 0;
 }