Enable HAVE_MADV_FREE_REUSE on iOS
https://bugs.webkit.org/show_bug.cgi?id=113892
<rdar://problem/12787563>

Reviewed by Mark Rowe.

* wtf/Platform.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147595 268f45cc-cd09-0410-ab3c-d52691b4dbfc