rust: bindgen: Add `alt_instr` as opaque type

To address this build error:

    BINDGEN rust/bindings/bindings_generated.rs
    BINDGEN rust/bindings/bindings_helpers_generated.rs
    EXPORTS rust/exports_core_generated.h
    RUSTC P rust/libmacros.so
    RUSTC L rust/compiler_builtins.o
    RUSTC L rust/alloc.o
    RUSTC L rust/bindings.o
    RUSTC L rust/build_error.o
    EXPORTS rust/exports_alloc_generated.h
  error[E0588]: packed type cannot transitively contain a `#[repr(align)]` type
       --> /var/home/acme/git/linux/rust/bindings/bindings_generated.rs:10094:1
        |
  10094 | / pub struct alt_instr {
  10095 | |     pub instr_offset: s32,
  10096 | |     pub repl_offset: s32,
  10097 | |     pub __bindgen_anon_1: alt_instr__bindgen_ty_1,
  10098 | |     pub instrlen: u8_,
  10099 | |     pub replacementlen: u8_,
  10100 | | }
        | |_^
        |
  note: `alt_instr__bindgen_ty_1__bindgen_ty_1` has a `#[repr(align)]` attribute
       --> /var/home/acme/git/linux/rust/bindings/bindings_generated.rs:10111:1
        |
  10111 | / pub struct alt_instr__bindgen_ty_1__bindgen_ty_1 {
  10112 | |     pub _bitfield_1: __BindgenBitfieldUnit<[u8; 4usize], u16>,
  10113 | | }
        | |_^
  note: `alt_instr` contains a field of type `alt_instr__bindgen_ty_1`
       --> /var/home/acme/git/linux/rust/bindings/bindings_generated.rs:10097:9
        |
  10097 |     pub __bindgen_anon_1: alt_instr__bindgen_ty_1,
        |         ^^^^^^^^^^^^^^^^
  note: ...which contains a field of type `alt_instr__bindgen_ty_1__bindgen_ty_1`
       --> /var/home/acme/git/linux/rust/bindings/bindings_generated.rs:10104:9
        |
  10104 |     pub __bindgen_anon_1: alt_instr__bindgen_ty_1__bindgen_ty_1,
        |         ^^^^^^^^^^^^^^^^

  error: aborting due to previous error

  For more information about this error, try `rustc --explain E0588`.
  make[1]: *** [rust/Makefile:389: rust/bindings.o] Error 1
  make: *** [Makefile:1293: prepare] Error 2

Cc: Derek Barbosa <debarbos@redhat.com>
Cc: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Fixes: 5d1dd961 ("x86/alternatives: Add alt_instr.flags")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>