rust: fix signature of rust_fmt_argument

rust: fix signature of rust_fmt_argument

Without this change, the rest of this series will emit the following
error message:

error[E0308]: `if` and `else` have incompatible types
  --> <linux>/rust/kernel/print.rs:22:22
   |
21 | #[export]
   | --------- expected because of this
22 | unsafe extern "C" fn rust_fmt_argument(
   |                      ^^^^^^^^^^^^^^^^^ expected `u8`, found `i8`
   |
   = note: expected fn item `unsafe extern "C" fn(*mut u8, *mut u8, *mut c_void) -> *mut u8 {bindings::rust_fmt_argument}`
              found fn item `unsafe extern "C" fn(*mut i8, *mut i8, *const c_void) -> *mut i8 {print::rust_fmt_argument}`

The error may be different depending on the architecture.

To fix this, change the void pointer argument to use a const pointer,
and change the imports to use crate::ffi instead of core::ffi for
integer types.

Fixes: 787983da7718 ("vsprintf: add new `%pA` format specifier")
Reviewed-by: Tamir Duberstein <tamird@gmail.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20250303-export-macro-v3-1-41fbad85a27f@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>