Define INVALID_FILE_ATTRIBUTES in filename.cpp too.

Define INVALID_FILE_ATTRIBUTES in filename.cpp too.

Fix VC6 compilation: INVALID_FILE_ATTRIBUTES is not defined in its headers so
we need to do it ourselves (this was done in filefn.cpp before but part of the
code using this constant was moved to filename.cpp in r62735 so now we need to
define it here too).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62737 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775