selftests/bpf: Remove extern from kfuncs declarations

There's no need to keep the extern in kfuncs declarations.

Suggested-by: David Vernet <void@manifault.com>
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230515133756.1658301-10-jolsa@kernel.org


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