add linux build tags to restrict compilation to linux targets

errors.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package systemctl
 
 import (

helpers.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package systemctl
 
 import (

structs.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package systemctl
 
 type Options struct {

systemctl.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package systemctl
 
 import (

util.go

@@ -1,3 +1,5 @@
+//go:build linux
+
 package systemctl
 
 import (