rust/wrapper.h: define memory_order enum

rust/wrapper.h: define memory_order enum

Add stub definition of memory_order enum in wrapper.h.

Creating Rust bindings from C code is done by passing the wrapper.h
header to `bindgen`. This fails when library dependencies that use
compiler headers are enabled, and the libclang that bindgen detects does
not match the expected clang version. So far this has only been observed
with the memory_order enum symbols from stdatomic.h. If we add the enum
definition to wrapper.h ourselves, the error does not happen.

Before this commit, if the mismatch happened the following error could
come up:

  /usr/include/liburing/barrier.h:72:10: error: use of undeclared identifier 'memory_order_release'
  /usr/include/liburing/barrier.h:75:9: error: use of undeclared identifier 'memory_order_acquire'
  /usr/include/liburing/barrier.h:75:9: error: use of undeclared identifier 'memory_order_acquire'
  /usr/include/liburing/barrier.h:68:9: error: use of undeclared identifier 'memory_order_relaxed'
  /usr/include/liburing/barrier.h:65:17: error: use of undeclared identifier 'memory_order_relaxed'
  /usr/include/liburing/barrier.h:75:9: error: use of undeclared identifier 'memory_order_acquire'
  /usr/include/liburing/barrier.h:75:9: error: use of undeclared identifier 'memory_order_acquire'
  /usr/include/liburing/barrier.h:72:10: error: use of undeclared identifier 'memory_order_release'
  panicked at [..]/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bindgen-cli-0.70.1/main.rs:45:36:
  Unable to generate bindings

To fix this (on my system) I would have to export CLANG_PATH and
LIBCLANG_PATH:

  export CLANG_PATH=/bin/clang-17
  export LIBCLANG_PATH=/usr/lib/llvm-17/lib

With these changes applied, bindgen is successful with both the
environment variables set and unset.

Since we're not using those symbols in the bindings (they are only used
by dependencies) this does not affect the generated bindings in any way.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/20241027-rust-wrapper-stdatomic-v2-1-dab27bbf93ea@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>