trivial fix, make cargo happy

Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
     "stm32-data-gen",
     "stm32-data-serde",

stm32-metapac-gen/res/build.rs

@@ -1,7 +1,9 @@
 use std::env;
+#[cfg(any(feature = "rt", feature = "memory-x"))]
 use std::path::PathBuf;
 
 fn main() {
+    #[cfg(any(feature = "rt", feature = "memory-x"))]
     let crate_dir = PathBuf::from(env::var_os("CARGO_MANIFEST_DIR").unwrap());
 
     let chip_core_name = env::vars_os()