Fix for <iostream.h> not found, using cpp11 Xcode

Hey,

I was trying out ofxLibwebsockets on [openFrameworks-cpp11/openFrameworks](https://github.com/openFrameworks-cpp11/openFrameworks). I am using xcode with cpp11. 

The only issue was that with this cpp11 repo has <iostream.h> in ofxLibwebsockets::Connections wasn't found. Fix provided below. 

I have tested this fix on of_v0.8.0_osx and all is ok. 
I have not testing on windows & linux but quite confident it should be ok.

Thanks,
Ross