rust: kernel: move `build_error` hidden function to prevent mistakes

Users were using the hidden exported `kernel::build_error` function
instead of the intended `kernel::build_error!` macro, e.g. see the
previous commit.

To force to use the macro, move it into the `build_assert` module,
thus making it a compilation error and avoiding a collision in the same
"namespace". Using the function now would require typing the module name
(which is hidden), not just a single character.

Now attempting to use the function will trigger this error with the
right suggestion by the compiler:

      error[E0423]: expected function, found macro `kernel::build_error`
      --> samples/rust/rust_minimal.rs:29:9
         |
      29 |         kernel::build_error();
         |         ^^^^^^^^^^^^^^^^^^^ not a function
         |
      help: use `!` to invoke the macro
         |
      29 |         kernel::build_error!();
         |                            +

An alternative would be using an alias, but it would be more complex
and moving it into the module seems right since it belongs there and
reduces the amount of code at the crate root.

Keep the `#[doc(hidden)]` inside `build_assert` in case the module is
not hidden in the future.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20241123222849.350287-2-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>