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
successfully).

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>
12 jobs for glib-tests-on-freebsd in 31 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Build
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build:cmake

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build:debug

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build:i686-w64-mingw32-cmake
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build:i686-w64-mingw32-debug

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build:legacy
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build:production

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build:reduced
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build:stretch
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build:x86_64-w64-mingw32

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passed #4165284
build:x86_64-w64-mingw32-cmake-debug

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build:xenial
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build:x86_64-w64-mingw32-cmake-debug

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