bpf: Define struct bpf_tcp_req_attrs when CONFIG_SYN_COOKIES=n.

kernel test robot reported the warning below:

  >> net/core/filter.c:11842:13: warning: declaration of 'struct bpf_tcp_req_attrs' will not be visible outside of this function [-Wvisibility]
      11842 |                                         struct bpf_tcp_req_attrs *attrs, int attrs__sz)
            |                                                ^
     1 warning generated.

struct bpf_tcp_req_attrs is defined under CONFIG_SYN_COOKIES
but used in kfunc without the config.

Let's move struct bpf_tcp_req_attrs definition outside of
CONFIG_SYN_COOKIES guard.

Fixes: e472f888

 ("bpf: tcp: Support arbitrary SYN Cookie.")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401180418.CUVc0hxF-lkp@intel.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240118211751.25790-1-kuniyu@amazon.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>