Fixed compilation of base64.h on clang + libc++

If `<ios>` is not included, the compilation fails with the following
error:

```
In file included from
ros_comm-1.9.47/utilities/xmlrpcpp/src/XmlRpcValue.cpp:5:
ros_comm-1.9.47/utilities/xmlrpcpp/include/base64.h:231:13:
error: incomplete type 'std::__1::ios_base' named in nested name
specifier
_St |= _IOS_FAILBIT|_IOS_EOFBIT; return _First; // unexpected EOF
^~~~~~~~~~~~
```