libopenbsd.rs(3): adds default value for Promises

src/libopenbsd.rs

@@ -43,6 +43,10 @@ impl Promises {
 	}
 }
 
+impl Default for Promises {
+	fn default() -> Self { Promises::new("") }
+}
+
 pub fn pledge(
 	promises: Option<Promises>, execpromises: Option<Promises>
 ) -> Result<(), Error> {