rust: workaround `bindgen` issue with forward references to `enum` types

rust: workaround `bindgen` issue with forward references to `enum` types

`bindgen` currently generates the wrong type for an `enum` when there
is a forward reference to it. For instance:

    enum E;
    enum E { A };

generates:

    pub const E_A: E = 0;
    pub type E = i32;

instead of the expected:

    pub const E_A: E = 0;
    pub type E = ffi::c_uint;

The issue was reported to upstream `bindgen` [1].

Now, both GCC and Clang support silently these forward references to
`enum` types, unless `-Wpedantic` is passed, and it turns out that some
headers in the kernel depend on them.

Thus, depending on how the headers are included, which in turn may depend
on the kernel configuration or the architecture, we may get a different
type on the Rust side for a given C `enum`.

That can be quite confusing, to say the least, especially since
developers may only notice issues when building for other architectures
like in [2]. In particular, they may end up forcing a cast and adding
an `#[allow(clippy::unnecessary_cast)]` like it was done in commit
94e05a66ea3e ("rust: hrtimer: allow timer restart from timer handler"),
which isn't great.

Instead, let's have a section at the top of our `bindings_helper.h` that
`#include`s the headers with the affected types -- hopefully there are
not many cases and there is a single ordering that covers all cases.

This allows us to remove the cast and the `#[allow]`, thus keeping the
correct code in the source files. When the issue gets resolved in upstream
`bindgen` (and we update our minimum `bindgen` version), we can easily
remove this section at the top.

Link: https://github.com/rust-lang/rust-bindgen/issues/3179
Link: https://lore.kernel.org/rust-for-linux/87tt7md1s6.fsf@kernel.org/
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://lore.kernel.org/r/20250325184309.97170-1-ojeda@kernel.org
[ Added extra paragraph on the comment to clarify that the workaround may
  not be possible in some cases. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>