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

[kavoc-brown]

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

poc.wit

package poc:cpp-span;

world w {
  import foo: func(x: list<u8>);
}

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)