samples: net: dumb_http_server: Add net_pkt.h file inclusion

The function net_pkt_get_info() is found in net_pkt.h and not
in buf.h. This caused
"implicit declaration of function ‘net_pkt_get_info’"
warning if CONFIG_NET_BUF_POOL_USAGE was enabled.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>