avfilter/avf_showvolume: use log10 instead of log()/M_LN10

This is likely more precise and conveys the intent better.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/avf_showvolume.c

@@ -197,7 +197,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *insamples)
             max = FFMAX(max, src[i]);
 
         max = av_clipf(max, 0, 1);
-        values[VAR_VOLUME] = 20.0 * log(max) / M_LN10;
+        values[VAR_VOLUME] = 20.0 * log10(max);
         values[VAR_CHANNEL] = c;
         color = av_expr_eval(s->c_expr, values, NULL);