Fix GNUStep linking

Author: madsmtm

objc2/benches/autorelease.rs

@@ -17,7 +17,12 @@ extern "C" {}
 
 const BYTES: &[u8] = &[1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
 
-fn empty() {}
+fn empty() {
+    #[cfg(gnustep)]
+    unsafe {
+        objc2::__gnustep_hack::get_class_to_force_linkage()
+    };
+}
 
 fn pool_cleanup() {
     autoreleasepool(|_| {})