fix: Fix compiler error found on latest main that breaks wasip1 (wasm) compilation

[scottmarchant]

This is a quick simple fix for compiling NIOCore to wasm using the command swift build --swift-sdk wasm32-unknown-wasip1-threads --target NIOCore.

This fixes all errors for wasip1, however there are other errors remaining in main when compiling for wasi using the command swift build --swift-sdk wasm32-unknown-wasi --target NIOCore. Those issues are out of scope for this PR.

Motivation:

I am working to enable wasm compilation for a wide range of repositories which depend on NIOCore. See here for a sneak peak. This change is required to enable compiling several respositories.

Modifications:

.multicastNotSupported is elided for os(WASI) builds in Channel.swift, however some new code referenced that case without proper WASI guards.

This modification simply adds the appropriate elision guard to fix wasip1 builds for NIOCore.

Result:

After making this change, swift build --swift-sdk wasm32-unknown-wasip1-threads --target NIOCore builds again (assuming you have the appropriate wasm sdk installed).

[gjcairo]

Thanks!

[scottmarchant]

@gjcairo Thanks for the review! Do you know what the process looks like to get this MR merged? I think a handful of other wasm-related efforts such as #3159 might become blocked waiting for this fix.