Don't probe system dependencies on Windows

This should help unbreak the Astra build on Windows.

build.rs

@@ -2,5 +2,7 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 fn main() {
+    // Windows doesn't ship pkgconfig files, typically. At least in our build system.
+    #[cfg(not(target_os = "windows"))]
     system_deps::Config::new().probe().unwrap();
 }