Update tests/fail-dep/concurrency/libc_pthread_mutex_move.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

tests/fail-dep/concurrency/libc_pthread_mutex_move.rs

@@ -11,7 +11,7 @@ fn check() {
         let mut m: libc::pthread_mutex_t = std::mem::zeroed();
         assert_eq!(libc::pthread_mutex_init(&mut m as *mut _, std::ptr::null()), 0);
 
-        let mut m2 = m;
+        let mut m2 = m; // move the mutex
         libc::pthread_mutex_lock(&mut m2 as *mut _); //~[init] ERROR: pthread_mutex_t can't be moved after first use
     }
 }