ffmpeg: use log10 instead of log()/log(10)

This is more concise and conveys the intent better.
Furthermore, it is likely more precise as well due to lack of floating
point division.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

ffmpeg.c

@@ -1244,7 +1244,7 @@ static void do_video_out(AVFormatContext *s,
 
 static double psnr(double d)
 {
-    return -10.0 * log(d) / log(10.0);
+    return -10.0 * log10(d);
 }
 
 static void do_video_stats(OutputStream *ost, int frame_size)