Guard with cfg...

cargo-test-sbf (governance/addin-mock/program, governance/program):
error[E0658]: use of unstable library feature 'once_cell'
   --> src/abi_example.rs:559:36
     |
 559 | impl<T: AbiExample> AbiExample for std::sync::OnceLock<T> {
     |                                    ^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: see issue #74465 <rust-lang/rust#74465> for more information
     = help: add `#![feature(once_cell)]` to the crate attributes to enable

frozen-abi/src/abi_example.rs

@@ -556,6 +556,7 @@ impl<O: AbiEnumVisitor, E: AbiEnumVisitor> AbiEnumVisitor for Result<O, E> {
     }
 }
 
+#[cfg(not(target_os = "solana"))]
 impl<T: AbiExample> AbiExample for std::sync::OnceLock<T> {
     fn example() -> Self {
         Self::from(T::example())