tests: On Unix, include <netinet/in.h> for IPPROTO_TCP

Otherwise, dbus doesn't compile on FreeBSD if the GLib-based tests
are enabled (which suggests that no FreeBSD user has run those tests

We already include <netinet/in.h> in other places with no conditions
or checks other than "is Unix", so apparently it's portable enough that
specifically testing for its presence is not necessary. POSIX requires it
to exist.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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