lavf: use a fixed width type

It's shorter and more consistent with the rest of the code.

libavformat/utils.c

@@ -243,7 +243,7 @@ int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt,
                           unsigned int offset, unsigned int max_probe_size)
 {
     AVProbeData pd = { filename ? filename : "" };
-    unsigned char *buf = NULL;
+    uint8_t *buf = NULL;
     int ret = 0, probe_size;
 
     if (!max_probe_size) {