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: NAnders Roxell <anders.roxell@linaro.org>
Tested-by: NDaniel Díaz <daniel.diaz@linaro.org>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Acked-by: NShuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/.gitignore

@@ -11,3 +11,4 @@ test_progs
 test_tcpbpf_user
 test_verifier_log
 feature
+test_libbpf_open