avformat/format: silence -Wdiscarded-qualifiers

lpd.buf is non-const and discards the const qualifier of zerobuffer.
This fixes -Wdiscarded-qualifiers observed with a variety of compilers, including GCC 5.2.
Note that this does not change the type of zerobuffer, and merely makes the intent explicit.
Signed-off-by: NGanesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: NMichael Niedermayer <michael@niedermayer.cc>

libavformat/format.c

@@ -175,7 +175,7 @@ AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
     const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
 
     if (!lpd.buf)
-        lpd.buf = zerobuffer;
+        lpd.buf = (unsigned char *) zerobuffer;
 
     if (lpd.buf_size > 10 && ff_id3v2_match(lpd.buf, ID3v2_DEFAULT_MAGIC)) {
         int id3len = ff_id3v2_tag_len(lpd.buf);