configury: add sys/socket.h to other socket header checks.

The net/if.h check fails on at least OpenBSD 5.6 without this
patch.
* configure.ac (AC_CHECK_HEADERS): Add sys/socket.h to default
headers when checking for net/if.h and linux/netlink.h.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>