uapi: net: ipv6: Replace fake flex-array with flex-array member

Zero-length arrays as fake flexible arrays are deprecated and we are
moving towards adopting C99 flexible-array members instead.

Address the following warning found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
net/ipv6/exthdrs.c: In function ‘fl6_update_dst’:
net/ipv6/exthdrs.c:1393:28: warning: array subscript 0 is outside array bounds of ‘struct in6_addr[0]’ [-Warray-bounds=]
 1393 |                 fl6->daddr = *((struct rt0_hdr *)opt->srcrt)->addr;
      |                 ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from ./include/linux/ipv6.h:5,
                 from ./include/linux/icmpv6.h:6,
                 from net/ipv6/exthdrs.c:27:
./include/uapi/linux/ipv6.h:84:33: note: while referencing ‘addr’
   84 |         struct in6_addr         addr[0];
      |                                 ^~~~
net/ipv6/exthdrs.c: In function ‘ipv6_push_rthdr0.isra’:
net/ipv6/exthdrs.c:1125:19: warning: array subscript <unknown> is outside array bounds of ‘struct in6_addr[0]’ [-Warray-bounds=]
 1125 |         phdr->addr[hops - 1] = **addr_p;
      |         ~~~~~~~~~~^~~~~~~~~~
./include/uapi/linux/ipv6.h:84:33: note: while referencing ‘addr’
   84 |         struct in6_addr         addr[0];
      |                                 ^~~~

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/276
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

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