net: gso_test: fix build with gcc-12 and earlier

net: gso_test: fix build with gcc-12 and earlier

gcc 12 errors out with:
net/core/gso_test.c:58:48: error: initializer element is not constant
   58 |                 .segs = (const unsigned int[]) { gso_size },

This version isn't old (2022), so switch to preprocessor-bsaed constant
instead of 'static const int'.

Cc: Willem de Bruijn <willemb@google.com>
Reported-by: Tasmiya Nalatwad <tasmiya@linux.vnet.ibm.com>
Closes: https://lore.kernel.org/netdev/79fbe35c-4dd1-4f27-acb2-7a60794bc348@linux.vnet.ibm.com/
Fixes: 1b4fa28a8b07 ("net: parametrize skb_segment unit test to expand coverage")
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20231012120901.10765-1-fw@strlen.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>