panic_qr: use new #[export] macro

This validates at compile time that the signatures match what is in the
header file. It highlights one annoyance with the compile-time check,
which is that it can only be used with functions marked unsafe.

If the function is not unsafe, then this error is emitted:

error[E0308]: `if` and `else` have incompatible types
   --> <linux>/drivers/gpu/drm/drm_panic_qr.rs:987:19
    |
986 | #[export]
    | --------- expected because of this
987 | pub extern "C" fn drm_panic_qr_max_data_size(version: u8, url_len: usize) -> usize {
    |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected unsafe fn, found safe fn
    |
    = note: expected fn item `unsafe extern "C" fn(_, _) -> _ {kernel::bindings::drm_panic_qr_max_data_size}`
               found fn item `extern "C" fn(_, _) -> _ {drm_panic_qr_max_data_size}`

The signature declarations are moved to a header file so it can be
included in the Rust bindings helper, and the extern keyword is removed
as it is unnecessary.

Reviewed-by: Andreas Hindborg <[email protected]>
Reviewed-by: Tamir Duberstein <[email protected]>
Acked-by: Simona Vetter <[email protected]>
Acked-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Reviewed-by: Jocelyn Falempe <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Fixed `rustfmt`. Moved on top the unsafe requirement comment to follow
  the usual style, and slightly reworded it for clarity. Formatted
  bindings helper comment. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

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