selftests/bpf: Add btf_dump__new to test_cpp

Adding btf_dump__new call to test_cpp, so we can
test C++ compilation with that.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211223131736.483956-2-jolsa@kernel.org