rust: add --rust-target option for bindgen

Without it, recent bindgen will give an error

   error: extern block cannot be declared unsafe

if rustc is not new enough to support the "unsafe extern" construct.

Cc: qemu-rust@nongnu.org
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20250206111514.2134895-1-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

meson.build

@@ -4054,6 +4054,9 @@ if have_rust
       bindgen_args += ['--formatter', 'none']
     endif
   endif
+  if bindgen.version().version_compare('>=0.66.0')
+    bindgen_args += ['--rust-target', '1.59']
+  endif
   if bindgen.version().version_compare('<0.61.0')
     # default in 0.61+
     bindgen_args += ['--size_t-is-usize']