avcodec/zmbvenc: use log2 instead of log()/M_LN2

This is likely more precise and conveys the intent better.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavcodec/zmbvenc.c

@@ -277,7 +277,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
     int lvl = 9;
 
     for(i=1; i<256; i++)
-        score_tab[i]= -i * log(i/(double)(ZMBV_BLOCK*ZMBV_BLOCK)) * (256/M_LN2);
+        score_tab[i]= -i * log2(i/(double)(ZMBV_BLOCK*ZMBV_BLOCK)) * 256;
 
     c->avctx = avctx;