Add main function to ui tests without main

These emit a new error not relevant to Serde as of nightly-2018-12-29.

test_suite/tests/ui/precondition/deserialize_de_lifetime.rs

@@ -5,3 +5,5 @@ extern crate serde_derive;
 struct S<'de> {
     s: &'de str,
 }
+
+fn main() {}

test_suite/tests/ui/precondition/deserialize_dst.rs

@@ -6,3 +6,5 @@ struct S {
     string: String,
     slice: [u8],
 }
+
+fn main() {}