fix(build): Add small doc struct to make rustdocs work on MacOS

This just adds a dummy struct of OCIBuildService to make rustdoc build
work on non-linux targets.

Change-Id: Iea958d9ed7ac0720ffe199c5c201e7a2edbba3ed
Reviewed-on: https://cl.snix.dev/c/snix/+/30649
Autosubmit: Brian Olsen <brian@maven-group.org>
Reviewed-by: Florian Klink <flokli@flokli.de>
Tested-by: besadii

snix/build/src/buildservice/from_addr.rs

@@ -4,6 +4,8 @@ use url::Url;
 
 #[cfg(target_os = "linux")]
 use super::oci::OCIBuildService;
+#[cfg(all(not(target_os = "linux"), doc))]
+struct OCIBuildService;
 
 /// Constructs a new instance of a [BuildService] from an URI.
 ///