net: slip: allow compilation without CONFIG_NET_LLDP

Without this change compilation of the SLIP module would fail if LLDP
wasn't enabled. There is also no need to include net/lldp.h explicitly
as net/ethernet.h will include it conditionally if CONFIG_NET_LLDP is
defined.

Signed-off-by: Sören Tempel <soeren+git@soeren-tempel.net>