libstdc++: vxworks: remove stray <iostream> include
There doesn't seem to be any reason for this TU to include
<iostream>, and it causes errors when the resulting libstdc++ is used
on our VxWorks 5.5 target - presumably because now libstdc++ itself
contains an instance of std::ios_base::Init. Which should be mostly
harmless, but apparently isn't, and from a QoI viewpoint should
probably be avoided anyway.