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: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NStanislav Fomichev <sdf@google.com>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Acked-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/.gitignore

@@ -42,3 +42,4 @@ xdping
 test_sockopt
 test_sockopt_sk
 test_sockopt_multi
+test_tcp_rtt