valgrind: Update `POSIX soc no cpu cleanup` suppression

Add `possible` to match-leak-kinds to prevent false positives caused by
POSIX soc no cpu cleanup.

The leak can be reproduced by adding CONFIG_NETWORKING=y to
tests/lib/cpp/libcxx/prj.conf and run twister:
twister -p native_sim -s tests/lib/cpp/libcxx/cpp.libcxx.host
--enable-valgrind

Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>