cpp: missing `<span>` include for `list<T>` parameters

When a WIT world imports a function that takes a `list` parameter, `wit-bindgen-cpp` generates C++ that uses `std::span<...>` in the public header but does not include ``. The generated bindings are not self-contained and fail to compile.

poc.wit
```
package poc:cpp-span;

world w {
import foo: func(x: list);
}
```

Steps:
1) Generate C++ bindings
2) Compile to see the error: `clang++ -c out/w.cpp -Iout -o out/w.o`

Actual result:
```
error: no member named 'span' in namespace 'std'
```

wit-bindgen version: 0.50.0

OS: Ubuntu 22.04.5 LTS (x86_64)