Enclose the 'include systemd' in the module

When using a different module to call the systemd
class and also using systemd::unit_file you get a
duplicate resource. By this 'if !defined' this is
solved. Issue #505

manifests/daemon_reload.pp

@@ -31,7 +31,9 @@
   Boolean $enable = true,
   Optional[String[1]] $user = undef,
 ) {
-  include systemd
+  if !defined(Class['systemd']) {
+    include systemd
+  }
 
   if $enable {
     if $user {

manifests/dropin_file.pp

@@ -35,7 +35,9 @@
   Boolean                                     $notify_service          = true,
   Boolean                                     $daemon_reload           = true,
 ) {
-  include systemd
+  if !defined(Class['systemd']) {
+    include systemd
+  }
 
   if $target {
     $_ensure = 'link'

manifests/network.pp

@@ -24,7 +24,9 @@
   Boolean                        $show_diff       = true,
   Boolean                        $restart_service = true,
 ) {
-  include systemd
+  if !defined(Class['systemd']) {
+    include systemd
+  }
 
   if $restart_service and $systemd::manage_networkd and $facts['systemd_internal_services'] and $facts['systemd_internal_services']['systemd-networkd.service'] {
     $notify = Service['systemd-networkd']

manifests/service_limits.pp

@@ -41,7 +41,9 @@
     fail('The restart_service parameter is deprecated and only false is a valid value')
   }
 
-  include systemd
+  if !defined(Class['systemd']) {
+    include systemd
+  }
 
   if $name !~ Pattern['^.+\.(service|socket|mount|swap)$'] {
     fail('$name must match Pattern["^.+\.(service|socket|mount|swap)$"]')

manifests/unit_file.pp

@@ -86,8 +86,10 @@
   Boolean                                  $daemon_reload = true,
   Boolean                                  $service_restart = true,
 ) {
-  include systemd
-
+  if !defined(Class['systemd']) {
+    include systemd
+  }
+
   assert_type(Systemd::Unit, $name)
 
   if $enable == 'mask' {