ar71xx: fix packed-not-aligned error with GCC 8.

Building ar71xx currently fails with:

    In file included from ./include/linux/ipv6.h:5,
                     from ./include/net/ipv6.h:16,
                     from ./include/net/inetpeer.h:16,
                     from ./include/net/ip_fib.h:24,
                     from ./include/net/switchdev.h:17,
                     from ./include/net/dsa.h:23,
                     from arch/mips/ath79/dev-dsa.h:15,
                     from arch/mips/ath79/dev-dsa.c:17:
    ./include/uapi/linux/ipv6.h:107:1: error: alignment 1 of 'struct ipv6_destopt_hao' is less than 2 [-Werror=packed-not-aligned]
     } __attribute__((packed));

Address this issue by correcting the alignment of the struct packing
pragma accordingly.

Fixes: FS#1805
Reported-by: Pascal Ernster <git@hardfalcon.net>
[reword subject, rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>