wrc_ptp_ppsi.o was not picked any more from libarch.a into ppsi.o,
now that we don't refer to pp_diag_verbosity any more. So force
linking it, or the final wrc.elf build will fail for undefined symbols.

Also, remove a no-more-needed -I directive.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>