ipv6: include ipv6 checksum header

The modules calls csum_ipv6_magic that's defined on ip6_checksum.h

This is part of the in-tree fixes

core/rtw_br_ext.c

@@ -49,6 +49,7 @@
 #include <linux/icmpv6.h>
 #include <net/ndisc.h>
 #include <net/checksum.h>
+#include <net/ip6_checksum.h>
 #endif
 #endif