Fix compilation for ARM64 arch

The kernel module failed to build
on arm64 platform. Other platforms like x86 can build without problem
because the csum_ipv6_magic function are defined in arch specific
asm/checksum.h, which is missing on arm64 platform.

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