selftests/net: add tls to .gitignore

Add the tls binary to .gitignore

Fixes: 7f657d5b ("selftests: tls: add selftests for TLS sockets")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>