selftests/bpf: Fix a selftest compilation error

selftests/bpf: Fix a selftest compilation error

When building the kernel and selftest with clang compiler (llvm17 or llvm18),
I hit the following compilation failure:
  In file included from progs/test_lwt_redirect.c:3:
  In file included from /usr/include/linux/ip.h:21:
  In file included from /usr/include/asm/byteorder.h:5:
  In file included from /usr/include/linux/byteorder/little_endian.h:13:
  /usr/include/linux/swab.h:136:8: error: unknown type name '__always_inline'
    136 | static __always_inline unsigned long __swab(const unsigned long y)
        |        ^
  /usr/include/linux/swab.h:171:8: error: unknown type name '__always_inline'
    171 | static __always_inline __u16 __swab16p(const __u16 *p)
  ...

bpf_helpers.h file provided a definition for __always_inline.
Putting 'ip.h' after 'bpf_helpers.h' fixed the issue.

Fixes: 43a7c3ef8a15 ("selftests/bpf: Add lwt_xmit tests for BPF_REDIRECT")
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230818174312.1883381-1-yonghong.song@linux.dev
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>