selftests/bpf: use __bpf_constant_htons in test_prog.c

For some reason, my older GCC (< 4.8) isn't smart enough to optimize the
!__builtin_constant_p() branch in bpf_htons, I see:
  error: implicit declaration of function '__builtin_bswap16'

Let's use __bpf_constant_htons as suggested by Daniel Borkmann.

I tried to use simple htons, but it produces the following:
  test_progs.c:54:17: error: braced-group within expression allowed only
  inside a function
    .eth.h_proto = htons(ETH_P_IP),

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>