fix(genpass): use `log()` instead of `log2()` for zsh < 5.6 (#9548)

Fixes #9548

plugins/genpass/genpass-xkcd

@@ -43,7 +43,7 @@ fi
 
 # Figure out how many words we need for 128 bits of security margin.
 # Each word adds log2($#words) bits.
-local -i n=$((ceil(128. / log2($#words))))
+local -i n=$((ceil(128. / (log($#words) / log(2)))))
 
 {
   local c