meson: Disable unsafe_attr_outside_unsafe for now

The #[unsafe(no_mangle)] decoration breaks cbindgen until 0.28 where
they added support.  Just disable that for now so that NIL still builds.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36849>

meson.build

@@ -814,9 +814,11 @@ if with_gallium_rusticl or with_nouveau_vk or with_tools.contains('etnaviv') or
     '-Dnever_type_fallback_flowing_into_unsafe',
     '-Drust_2024_prelude_collisions',
     '-Dstatic_mut_refs',
-    '-Dunsafe_attr_outside_unsafe',
     '-Dunsafe_op_in_unsafe_fn',
 
+    # This requires cbindgen >= 0.28
+    # '-Dunsafe_attr_outside_unsafe',
+
     # 1.83+
     # '-Dif_let_rescope',
     # '-Drust_2024_guarded_string_incompatible_syntax',