selftests/bpf: Fix compilation failure for prog vrf_socket_lookup

When building the latest kernel/selftest with clang17 compiler:

    make LLVM=1 -j                                  <== for kernel
    make -C tools/testing/selftests/bpf LLVM=1 -j   <== for selftest

I hit the following compilation error:

  [...]
  In file included from progs/vrf_socket_lookup.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)
        |        ^
  /usr/include/linux/swab.h:171:29: error: expected ';' after top level declarator
    171 | static __always_inline __u16 __swab16p(const __u16 *p)
        |                             ^
  [...]

Basically, with header files in my local host which is based on 5.12 kernel,
__always_inline is not defined and this caused compilation failure.

Since __always_inline is defined in bpf_helpers.h, let us move bpf_helpers.h
to an early position which fixed the problem.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230622061921.816772-1-yhs@fb.com