core/rtw_br_ext: include definition of csum_ipv6_magic

Fixes the following error observed while compiling this driver on arm64.

core/rtw_br_ext.c:1447:9: error: implicit declaration of function
‘csum_ipv6_magic’ [-Werror=implicit-function-declaration]

Signed-off-by: Tyler Baker <tyler.baker@linaro.org>

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