selftests: bpf: update .gitignore with missing file

Fixes: c0fa1b6c3efc ("bpf: btf: Add BTF tests")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/.gitignore

@@ -12,3 +12,4 @@ test_tcpbpf_user
 test_verifier_log
 feature
 test_libbpf_open
+test_btf