Workaround historic linux/netfilter/xt_osf.h deficiencies

Workaround historic linux/netfilter/xt_osf.h deficiencies

When using linux kernel headers without commit v4.5-rc1~128^2~115^2~22
("netfilter: fix include files for compilation"), both <linux/ip.h>
and <linux/tcp.h> have to be included before <linux/netfilter/xt_osf.h>.

* configure.ac (AC_CHECK_HEADERS): Include <linux/ip.h>
and <linux/tcp.h> when checking for linux/netfilter/xt_osf.h header.
* tests/nfnetlink_osf.c: Include <linux/ip.h> and <linux/tcp.h>
before <linux/netfilter/xt_osf.h>.