Fix compiler warnings in example projects

[simonjbeaumont]

Motivation

A recent example project was added that had a compiler warning which wasn't caught by CI. We can have our example code free of warnings and ensure they remain so by compiling them with -warnings-as-errors in CI.

On further investigation, we cannot enable -warnings-as-errors in the examples Ci pipeline because we're reusing the shared build/cache directories which produces its own warnings for left over unused modules.

Modifications

• Fix compiler warnings in example.
• Build with -warnings-as-errors in examples CI pipeline.

Result

Example projects compile without warnings.

Test Plan

CI updated.

[simonjbeaumont]

Oh, this is annoying:

[16/19] Compiling HelloWorldURLSessionClient Client.swift
/tmp/test-examples.sh.mM8MYEM7LH/swift-openapi-example-cache/x86_64-unknown-linux-gnu/debug/CBacktrace.build/module.modulemap:2:14: error: umbrella directory '/tmp/test-examples.sh.mM8MYEM7LH/swift-openapi-example-cache/checkouts/swift-backtrace/Sources/CBacktrace/include' not found
    umbrella "/tmp/test-examples.sh.mM8MYEM7LH/swift-openapi-example-cache/checkouts/swift-backtrace/Sources/CBacktrace/include"

This is leftover because we're using the shared cache/build dirs, which produces a warning. I guess we cannot enable -warnings-as-errors in CI then.

Will update PR to just fix the code to remove the compiler warning instead.