selftests/bpf: Make sure enum-less bpf_enable_stats() API works in C++ mode

Just a simple test to make sure we don't introduce unwanted compiler
warnings and API still supports passing enums as input argument.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221130200013.2997831-2-andrii@kernel.org