selftests/bpf: add missing executables to .gitignore

We build test_libbpf with CXX to make sure linking against C++ works.

$ make -s -C tools/lib/bpf
$ git status -sb
? tools/lib/bpf/test_libbpf
$ make -s -C tools/testing/selftests/bpf
$ git status -sb
? tools/lib/bpf/test_libbpf
? tools/testing/selftests/bpf/test_libbpf

Fixes: 8c4905b995c6 ("libbpf: make sure bpf headers are c++ include-able")
Signed-off-by: Stanislav Fomichev <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>

tools/lib/bpf/.gitignore

···
 libbpf_version.h
 FEATURE-DUMP.libbpf
+test_libbpf

tools/testing/selftests/bpf/.gitignore

···
 test_netcnt
 test_section_names
 test_tcpnotify_user
+test_libbpf