selftests/bpf: Allow building bpf tests with CONFIG_XFRM_INTERFACE=[m|n]

selftests/bpf: Allow building bpf tests with CONFIG_XFRM_INTERFACE=[m|n]

It is useful to use vmlinux.h in the xfrm_info test like other kfunc
tests do.  In particular, it is common for kfunc bpf prog that requires
to use other core kernel structures in vmlinux.h

Although vmlinux.h is preferred, it needs a ___local flavor of
struct bpf_xfrm_info in order to build the bpf selftests
when CONFIG_XFRM_INTERFACE=[m|n].

Cc: Eyal Birger <eyal.birger@gmail.com>
Fixes: 90a3a05eb33f ("selftests/bpf: add xfrm_info tests")
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20221206193554.1059757-1-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>