selftests/bpf: Fix selftests build with old system-wide headers

migrate_reuseport.c selftest relies on having TCP_FASTOPEN_CONNECT defined in
system-wide netinet/tcp.h. Selftests can use up-to-date uapi/linux/tcp.h, but
that one doesn't have SOL_TCP. So instead of switching everything to uapi
header, add #define for TCP_FASTOPEN_CONNECT to fix the build.

Fixes: c9d0bdef ("bpf: Test BPF_SK_REUSEPORT_SELECT_OR_MIGRATE.")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/bpf/20210617041446.425283-1-andrii@kernel.org