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

The bpf-nf selftests play various games with aliased types such that
folks with CONFIG_NF_CONNTRACK=m/n configs can still build the
selftests. See commits:

1058b6a7 ("selftests/bpf: Do not fail build if CONFIG_NF_CONNTRACK=m/n")
92afc532 ("selftests/bpf: Fix build errors if CONFIG_NF_CONNTRACK=m")

Thus, it is simpler if these selftests opt out of using generated 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/044a5b10cb3abd0d71cb1c818ee0bfc4a2239332.1718207789.git.dxu@dxuuu.xyz


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