We customarily #include <netinet/in.h> before <arpa/inet.h>; according
to our git history (cf commit 527f8bab) there used to be platform(s)
where <arpa/inet.h> didn't compile otherwise.  That's probably not
really an issue anymore, but since test_ifaddrs.c is the one and only
place in our code that's not following that rule, bring it into line.
Also remove #include <sys/socket.h>, as that's duplicative given that
libpq/ifaddr.h does so (via pqcomm.h).

In passing, add a .gitignore file so nobody accidentally commits the
test_ifaddrs executable, as I nearly did.

I see no particular need to back-patch this, as it's just neatnik-ism
considering we don't build test_ifaddrs by default, or even document
it anywhere.