for-ppsi: link libstd, too

In recent ppsi code, proto-ext-whiterabbit is not replicating
proto-standard any more, but rather it offers hooks for the additional
code it needs.  Thus, we need to link the library for standard PTP as
well, to pick some of the states for the WR state machine.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>