samples/zoap_server: 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: Ie1dacd07eb5a0891a572b08268a865f05a373690
Signed-off-by: Vinicius Costa Gomes <[email protected]>

Author: vcgomes

samples/net/zoap_server/prj.conf

@@ -4,7 +4,7 @@ CONFIG_NET_IPV6=y
 CONFIG_NET_LOG=y
 CONFIG_NET_SLIP=y
 CONFIG_NET_UDP=y
-CONFIG_SLIP_TAP=y
+CONFIG_NET_SLIP_TAP=y
 CONFIG_SYS_LOG=y
 CONFIG_SYS_LOG_SHOW_COLOR=y
 CONFIG_RANDOM_GENERATOR=y