Add omniORB4::sslTP4 target
To use the TLS / SSL transport, linking against the omnisslTP library is required.
This was tested in @tjuerges branch hackathon/use-tls-in-omniorb
If we want to use the latest version of this repo with older version of cpptango, I guess we should not make omniORB4_sslTP4_FOUND
always required. Not sure what's the best way to do that.