fix(.gitignore): un-ignore generated/mod.rs

The root `generated/` pattern matched guest-api/src/generated/mod.rs and
host-api/src/generated/mod.rs, but those files are committed stub modules
referenced from each crate's lib.rs. The tracked-yet-gitignored state
broke release-plz's cargo invocations ("the working directory has
uncommitted changes").

Switch to `**/generated/*` + `!**/generated/mod.rs` so the bulk of
prpc-build output stays ignored while committed mod.rs files are
recognized as clean.

Author: kvinwang

.gitignore

@@ -3,7 +3,8 @@
 /build-config.sh
 /build/*
 !/build/shared/
-generated/
+**/generated/*
+!**/generated/mod.rs
 node_modules/
 /.cargo
 .venv