Avoid using log2() freebsd does not support it.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13184 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavcodec/zmbvenc.c

@@ -246,7 +246,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
     int lvl = 9;
 
     for(i=1; i<256; i++)
-        score_tab[i]= -i * log2(i/(double)(ZMBV_BLOCK*ZMBV_BLOCK)) * 256;
+        score_tab[i]= -i * log(i/(double)(ZMBV_BLOCK*ZMBV_BLOCK)) * (256/M_LN2);
 
     c->avctx = avctx;