Xcode project: src/regex must be in non-user search path

Otherwise it wouldn't be included as <regex.h> and the system copy would
be used. We need to always use the builtin, wxChar-aware copy.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78217 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775