Add 'pid' to socket peercred on FreeBSD

The 'pid' of the socket peer was added to xucred in FreeBSD 12.

Lttng-tools doesn't yet have full support for FreeBSD but once it's
merged the baseline should be either FreeBSD 12 or 13 so we can get away
with no compat code.

Change-Id: I238baef7eedb3a909fb3804e6fd2cca4db84b21a
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>