ChangeLog: about malloc_size(3) availability

* ChangeLog: malloc_size(3) on MacOS X says it's a BSD Library
  Function, so possibly some other BSDs may have but seems unavailable
  on FreeBSD at least.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44004 b2dd03c8-39d4-4d8f-98ff-823fe69b080e