Avoid using log2, it is not available everywhere.

Should fix compilation on FreeBSD.

Originally committed as revision 21942 to svn://svn.ffmpeg.org/ffmpeg/trunk

ffprobe.c

@@ -67,7 +67,7 @@ static char *value_string(char *buf, int buf_size, double val, const char *unit)
         int index;
 
         if (unit == unit_byte_str && use_byte_value_binary_prefix) {
-            index = (int) (log2(val)) / 10;
+            index = (int) (log(val)/log(2)) / 10;
             index = av_clip(index, 0, FF_ARRAY_ELEMS(binary_unit_prefixes) -1);
             val /= pow(2, index*10);
             prefix_string = binary_unit_prefixes[index];