Clean stale wpa dependencies and objects after r341759

Clean stale wpa dependencies and objects after r341759

The wpa update added some source files with the same name as a file in
another directory (found via .PATH in the previous version).  Having a
stale entry in a .depend file means the new file won't be built, so test
for this case and if found remove all of wpa's dependency files.

MFC with:	r341759
Sponsored by:	The FreeBSD Foundation