Use postgres_backend feature flag for SupportsCombinationClause impl

[hgzimmerman]

I found that I couldn't compile queries that contained CombineDSL::union and CombineDSL::union_all expressions with the postgres feature flag 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. In a quick search along with personal experience from making the feature-flag switch in my project, I didn't come across other cases where postgres is used instead of postgres_backend as a feature-gate.

[Ten0]

I didn't come across other cases where postgres is used instead of postgres_backend as a feature-gate

I'm guessing the whole connection implementation based on libpq?

diesel/diesel/src/pg/mod.rs

Lines 11 to 12 in 7042083

	#[cfg(feature = "postgres")]
	pub(crate) mod connection;

[hgzimmerman]

Sorry, I was imprecise in my wording there. Aside from the contents of pg::connection, I didn't come across any other feature-flagged modules that impact Diesel's DSL with respect to postgres. The feature flag for "postgres" on the connection module is appropriate.

My intent was to remove my dependency on libpq by only using the implementation in tokio-postgres depended upon by diesel-async, but ran into this limitation.