logger/journald: simplify build constraints

Ensure the package can be imported, no matter the build constratints, by
adding an unconstrained doc.go containing a package statement.

Signed-off-by: Cory Snider <csnider@mirantis.com>

daemon/logger/journald/doc.go

@@ -0,0 +1,3 @@
+// Package journald provides the log driver for forwarding server logs
+// to endpoints that receive the systemd format.
+package journald // import "github.com/docker/docker/daemon/logger/journald"

daemon/logger/journald/journald.go

@@ -1,8 +1,6 @@
 //go:build linux
 // +build linux
 
-// Package journald provides the log driver for forwarding server logs
-// to endpoints that receive the systemd format.
 package journald // import "github.com/docker/docker/daemon/logger/journald"
 
 import (

daemon/logger/journald/journald_unsupported.go

@@ -1,7 +0,0 @@
-//go:build !linux
-// +build !linux
-
-package journald // import "github.com/docker/docker/daemon/logger/journald"
-
-type journald struct {
-}