net: enetc: include ip6_checksum.h for csum_ipv6_magic

For those architectures which do not define_HAVE_ARCH_IPV6_CSUM, we need
to include ip6_checksum.h which provides the csum_ipv6_magic() function.

Fixes: fb8629e2cbfc ("net: enetc: add support for software TSO")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Ioana Ciornei <[e-mail]>
Reviewed-by: Vladimir Oltean <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/freescale/enetc/enetc.c

···
 #include <linux/udp.h>
 #include <linux/vmalloc.h>
 #include <linux/ptp_classify.h>
+#include <net/ip6_checksum.h>
 #include <net/pkt_sched.h>
 #include <net/tso.h>