Add build.rs to link rstrtmgr on Windows

DuckDB 1.4+ uses the Windows Restart Manager API (RmStartSession, etc.)
which requires linking against rstrtmgr.lib. Without this, Windows
builds fail with LNK2019 unresolved externals.

Author: cpsievert

build.rs

@@ -0,0 +1,7 @@
+fn main() {
+    // DuckDB 1.4+ uses the Windows Restart Manager API, which lives in rstrtmgr.lib.
+    // Without this, linking fails with unresolved symbols for RmStartSession, etc.
+    if std::env::var("CARGO_CFG_TARGET_OS").as_deref() == Ok("windows") {
+        println!("cargo:rustc-link-lib=rstrtmgr");
+    }
+}