Make specification/loops.rs pass

Author: arnaudgolfouse

tests/should_succeed/specification/loops.rs

@@ -1,6 +1,7 @@
 extern crate creusot_contracts;
 use creusot_contracts::*;
 
+#[requires(!x)]
 pub fn while_loop_variant(x: bool) {
     #[variant(0)]
     while x {}