selftests/bpf: add test_tcp_rtt to .gitignore

Forgot to add it in the original patch.

Fixes: b5587398 ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
Reported-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>