TBR: Adds ios build tags to discovery APIs to fix non-iOS darwin builds.

Change-Id: I8f58f70305bc8a1e00322ef3bb90419febd5fb8d
diff --git a/v23/discovery/swift.go b/v23/discovery/swift.go
index 92c1ad5..199e0cd 100644
--- a/v23/discovery/swift.go
+++ b/v23/discovery/swift.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin
+// +build darwin,ios
 
 package discovery
 
diff --git a/v23/discovery/util.go b/v23/discovery/util.go
index 3b24c89..324a02e 100644
--- a/v23/discovery/util.go
+++ b/v23/discovery/util.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin
+// +build darwin,ios
 
 package discovery