IP6_HLEN is only defined when LWIP_IPV6, IP_HLEN is only defined when LWIP_IPV4.
This fixes build error in !LWIP_IPV4 || !LWIP_IPV6 cases.
Fixes: f2c7e9c939ed ("raw: Remove unnecessary #if guard around PCB_ISIPV6() calls")
Reported-by: Erik Ekman
Signed-off-by: Axel Lin <axel.lin@ingics.com>