selftests/bpf: fix compilation failure when CONFIG_NF_FLOW_TABLE=m

selftests/bpf: fix compilation failure when CONFIG_NF_FLOW_TABLE=m

In many cases, kernel netfilter functionality is built as modules.
If CONFIG_NF_FLOW_TABLE=m in particular, progs/xdp_flowtable.c
(and hence selftests) will fail to compile, so add a ___local
version of "struct flow_ports".

Fixes: c77e572d3a8c ("selftests/bpf: Add selftest for bpf_xdp_flow_lookup kfunc")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/r/20240710150051.192598-1-alan.maguire@oracle.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>