ipv4: tunnel: fix compilation on ARCH=um

With certain configurations, a 64-bit ARCH=um errors
out here with an unknown csum_ipv6_magic() function.
Include the right header file to always have it.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -38,6 +38,7 @@
 #include <net/geneve.h>
 #include <net/vxlan.h>
 #include <net/erspan.h>
+#include <net/ip6_checksum.h>
 
 const struct ip_tunnel_encap_ops __rcu *
 		iptun_encaps[MAX_IPTUN_ENCAP_OPS] __read_mostly;