added missing func.

nameservers.go

@@ -1,7 +1,14 @@
 package ctrld
 
+import "runtime"
+
 type dnsFn func() []string
 
+// isMobile reports whether the current OS is a mobile platform.
+func isMobile() bool {
+	return runtime.GOOS == "android" || runtime.GOOS == "ios"
+}
+
 // nameservers returns DNS nameservers from system settings.
 func nameservers() []string {
 	var dns []string