io.hpp is required to compile vector of dimensioned types

To reproduce:

```cpp
#include
#include
// Uncomment to fix:
//#include

using namespace boost::units;
using namespace boost::units::si;

auto dimensioned_sphere(std::vector> const & v) {
return v[0];
}

int main()
{
return 0;
}
```

Compile error:

```
main.cpp:9:10: fatal error: implicit instantiation of undefined template 'boost::units::quantity>, boost::units::dimensionless_type>, boost::units::homogeneous_system>>, boost::units::list>>>>>>>>>>>'
9 | return v[0];
| ^
~/boost/boost/units/units_fwd.hpp:52:45: note: template is declared here
52 | template class quantity;
| ^
1 error generated.
```

Error on top of 45787015dd8c11653eb988260acf05c4af9d42e5.