examples/vhost: remove IPv4 header definition
examples/vhost: remove IPv4 header definition

Remove the ipv4_hdr structure defination in vhost sample.

The same structure has already defined in the rte_ip.h file, so we
  remove the defination from the sample, and include that header file.

Signed-off-by: Jijiang Liu <10b95deab7df654d0c21c8c98b54c9a1ecf2a5c0@intel.com>
Acked-by: Yuanhan Liu <5dcef21195e64c9c08aa779bb62b25e9133eaa33@linux.intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk