netfilter: ipv6: using csum_ipv6_magic requires net/ip6_checksum.h

Fixes this build error:

net/ipv6/netfilter/nf_nat_l3proto_ipv6.c: In function 'nf_nat_ipv6_csum_recalc':
net/ipv6/netfilter/nf_nat_l3proto_ipv6.c:144:4: error: implicit declaration of function 'csum_ipv6_magic' [-Werror=implicit-function-declaration]
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/netfilter/nf_nat_l3proto_ipv6.c

@@ -15,6 +15,7 @@
 #include <linux/netfilter_ipv6.h>
 #include <net/secure_seq.h>
 #include <net/checksum.h>
+#include <net/ip6_checksum.h>
 #include <net/ip6_route.h>
 #include <net/ipv6.h>