selftests/bpf: add missing executables to .gitignore

Fixes: 371e4fcc ("selftests/bpf: cgroup local storage-based network counters")
Fixes: 370920c4 ("selftests/bpf: Test libbpf_{prog,attach}_type_by_name")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>