include <net/if.h> earlier

On FreeBSD using today's squid3-HEAD, I get a compile error in
src/store_dir.cc, where it complains about struct statfs being
undefined.

I tracked it down to failure of the autoconf test for whether
<sys/mount.h> was usable. Including <net/if.h> earlier in the
definition of autoconf's SQUID_BSDNET_INCLUDES macro fixes it; see
attached patch.

--apb (Alan Barrett)