bpf: selftests: xfrm: Opt out of using generated kfunc prototypes

The xfrm_info selftest locally defines an aliased type such that folks
with CONFIG_XFRM_INTERFACE=m/n configs can still build the selftests.
See commit aa67961f ("selftests/bpf: Allow building bpf tests with CONFIG_XFRM_INTERFACE=[m|n]").

Thus, it is simpler if this selftest opts out of using enerated kfunc
prototypes. The preprocessor macro this commit uses will be introduced
in the final commit.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/afe0bb1c50487f52542cdd5230c4aef9e36ce250.1718207789.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>