internal/postgres: close prepared statement in InsertVersion

stmt.Close() is called to free any resources used to create a prepared
statement for inserting packages inside postgres.InsertVersion.

Change-Id: I84ca3128af503b343f0b7f811090ddd8cb9226ab
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/440425
Reviewed-by: Andrew Bonventre <andybons@google.com>

internal/postgres/postgres.go

@@ -388,6 +388,7 @@ func (db *DB) InsertVersion(version *internal.Version) error {
 		if err != nil {
 			return fmt.Errorf("error preparing package stmt: %v", err)
 		}
+		defer stmt.Close()
 
 		for _, p := range version.Packages {
 			if _, err = stmt.Exec(p.Path, p.Synopsis, p.Name, version.Version, version.Module.Path, version.VersionType.String()); err != nil {