Enable feature flag for SupportsCombinationClause for postgres_backend

I found that I couldn't compile queries that contained union and union_all expressions with the `postgres` feature flag turned replaced with `postgres_backend` while switching to `diesel-async`.
I believe that this is a simple change to allow use of the `CombineDSL` trait with just the `postgres_backend` feature.

diesel/src/query_builder/combination_clause.rs

@@ -234,7 +234,7 @@ pub trait SupportsCombinationClause<Combinator, Rule> {}
 /// Wrapper used to wrap rhs sql in parenthesis when supported by backend
 pub struct ParenthesisWrapper<T>(T);
 
-#[cfg(feature = "postgres")]
+#[cfg(feature = "postgres_backend")]
 mod postgres {
     use super::*;
     use crate::pg::Pg;