src/poky:ppp: update 2.5.1 -> 2.5.2

License-Update: BSD items clarified to be 2-clause:
ppp-project/ppp@d4ec06e
passprompt plugin removed upstream.

Add an enable a multilink option, as the build breaks without it at the linking
step (upstream presumably didn't test that it works): ppp-project/ppp#541

(From OE-Core rev: e6ea3ade8f3977ec93c68229d2660bbc335a80a6)

Signed-off-by: Alexander Kanavin <alexlinutronix.de>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>