utils: add missing include flags to Makefile

Fixes this compile error:

  CC     nfnl_osf.o
nfnl_osf.c:48:36: fatal error: linux/netfilter/xt_osf.h: No such file or directory
compilation terminated.

References: http://marc.info/?l=netfilter&m=127449929621579&w=2
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
1 file changed