samples/zoap_client: Use the correct option for TAP

When using SLIP for TAP networking between guest and host, the correct
option is CONFIG_NET_SLIP_TAP. CONFIG_SLIP_TAP only enables it on the
SLIP side, it should be enabled on the networking stack, as well.

Change-Id: I76226db9c587f1b5e9613244adab8f916ad94013
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>