Disable wxUSE_PREFERENCES_EDITOR in correct place in wxiOS.

wx/osx/setup0.h is auto-generated, so r74708 shouldn't have changed it as the
change was overwritten by r75568 which regenerated that file. Instead, disable
wxUSE_PREFERENCES_EDITOR in iPhone-specific file.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75571 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775