Pass gecko-in-tree feature to cubeb-backend.

This lets us break a circular dependency when cubeb includes
cubeb-coreaudio-rs.

Cargo.toml

@@ -26,6 +26,7 @@ itertools = "0.11"
 
 [features]
 audio-dump = []
+gecko-in-tree = ["cubeb-backend/gecko-in-tree"]
 
 # Workaround for https://github.com/rust-lang/cargo/issues/6745 to allow this
 # Cargo.toml file to appear under a subdirectory of a workspace without being in