chore: require ffi feature in arrow-schema benchmark

[ethe]

Rationale for this change

setting required-features in [[bench]] makes rust-analyzer happy, otherwise error is linted as below:

error[E0432]: unresolved import `arrow_schema::ffi`
  --> arrow-schema/benches/ffi.rs:18:19
   |
18 | use arrow_schema::ffi::FFI_ArrowSchema;
   |                   ^^^ could not find `ffi` in `arrow_schema`
   |
note: found an item that was configured out
  --> /Users/gwo/Idea/arrow-rs/arrow-schema/src/lib.rs:44:9
   |
44 | pub mod ffi;
   |         ^^^
note: the item is gated behind the `ffi` feature
  --> /Users/gwo/Idea/arrow-rs/arrow-schema/src/lib.rs:43:7
   |
43 | #[cfg(feature = "ffi")]
   |       ^^^^^^^^^^^^^^^

What changes are included in this PR?

modified arrow-schema/Cargo.toml

Are there any user-facing changes?

no there aren't.

[ethe]

CI is failed but I don't think it relates to this change.

[alamb]

CI is failed but I don't think it relates to this change.

Agreed. I believe it is the same as

• Archery integration CI test is failing on main: error: package half v2.5.0 cannot be built because it requires rustc 1.81 or newer, while the currently active rustc version is 1.77.2 #7291

[alamb]

Thank you @ethe -- this makes sense to me