selftests/bpf: update gitignore with test_libbpf_open

bpf builds a test program for loading BPF ELF files. Add the executable
to the .gitignore list.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: Daniel Díaz <daniel.diaz@linaro.org>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>