URLSession transport fails to build for library evolution

[czechboy0]

URLSession transport fails to build for library evolution.

Try with swift build -Xswiftc -emit-module-interface -Xswiftc -enable-library-evolution.

While we don't guarantee ABI stability, it's still important to be able to build the project with library evolution enabled. Might we also worth adding it to our CI?

[czechboy0]

Fixed in apple/swift-openapi-urlsession#30