avformat/tests/movenc: Constify write functions

Forgotten in 2a68d945cd.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/tests/movenc.c

@@ -96,7 +96,7 @@ static void reset_count_warnings(void)
     av_log_set_callback(av_log_default_callback);
 }
 
-static int io_write(void *opaque, uint8_t *buf, int size)
+static int io_write(void *opaque, const uint8_t *buf, int size)
 {
     out_size += size;
     av_md5_update(md5, buf, size);
@@ -105,7 +105,7 @@ static int io_write(void *opaque, uint8_t *buf, int size)
     return size;
 }
 
-static int io_write_data_type(void *opaque, uint8_t *buf, int size,
+static int io_write_data_type(void *opaque, const uint8_t *buf, int size,
                               enum AVIODataMarkerType type, int64_t time)
 {
     char timebuf[30], content[5] = { 0 };