ip_tunnel_core: Fix build for archs without _HAVE_ARCH_IPV6_CSUM

On architectures defining _HAVE_ARCH_IPV6_CSUM, we get
csum_ipv6_magic() defined by means of arch checksum.h headers. On
other architectures, we actually need to include net/ip6_checksum.h
to be able to use it.

Without this include, building with defconfig breaks at least for
s390.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 4cb47a86 ("tunnels: PMTU discovery support for directly bridged IP packets")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>