Re-export cc's parallel feature for cxx-build

This allows building in parallel when the feature is enabled.
diff --git a/gen/build/Cargo.toml b/gen/build/Cargo.toml
index 36c7a31..98929db 100644
--- a/gen/build/Cargo.toml
+++ b/gen/build/Cargo.toml
@@ -11,6 +11,9 @@
 keywords = ["ffi"]
 categories = ["development-tools::ffi"]
 
+[features]
+parallel = ["cc/parallel"]
+
 [dependencies]
 cc = "1.0.49"
 codespan-reporting = "0.11"