Revert "avformat/format: silence -Wdiscarded-qualifiers"

This reverts commit 27cbe4588a.

See
https://lists.ffmpeg.org/pipermail/ffmpeg-devel/2015-September/179002.html

Requested-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>

libavformat/format.c

@@ -172,7 +172,7 @@ AVInputFormat *av_probe_input_format3(AVProbeData *pd, int is_opened,
     AVProbeData lpd = *pd;
     AVInputFormat *fmt1 = NULL, *fmt;
     int score, nodat = 0, score_max = 0;
-    static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
+    const static uint8_t zerobuffer[AVPROBE_PADDING_SIZE];
 
     if (!lpd.buf)
         lpd.buf = zerobuffer;