libopenjpeg: Drop const qualifier to silence compiler warning.

libavcodec/libopenjpeg.c💯 warning: passing argument 2 of ‘opj_cio_open’ discards qualifiers from pointer target type
/usr/include/openjpeg.h:770: note: expected ‘unsigned char *’ but argument is of type ‘const uint8_t *’

libavcodec/libopenjpeg.c

@@ -61,7 +61,7 @@ static int libopenjpeg_decode_frame(AVCodecContext *avctx,
                                     void *data, int *data_size,
                                     AVPacket *avpkt)
 {
-    const uint8_t *buf = avpkt->data;
+    uint8_t *buf = avpkt->data;
     int buf_size = avpkt->size;
     LibOpenJPEGContext *ctx = avctx->priv_data;
     AVFrame *picture = &ctx->image, *output = data;