examples/multi_process: fix FreeBSD build

examples/multi_process: fix FreeBSD build

Caught on FreeBSD 12:

/usr/include/netinet/ip.h:71:17: error: field 'ip_src' has incomplete type
  struct in_addr ip_src,ip_dst; /* source and dest address */
                 ^~~~~~

On FreeBSD, netinet/ip.h is not auto sufficient like on Linux.
But actually, this header is not used in the example, just remove it.

Fixes: 764bf26873b9 ("add FreeBSD support")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>