selftests/bpf: add test_tcp_rtt to .gitignore

Forgot to add it in the original patch.

Fixes: b558739 ("selftests/bpf: test BPF_SOCK_OPS_RTT_CB")
Reported-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Stanislav Fomichev <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

Author: fomichev

tools/testing/selftests/bpf/.gitignore

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