Support "none" for stl when using ndk

Change-Id: I13ecf0a6be591814546a9a1c8b8356988348feb0
diff --git a/cc/stl.go b/cc/stl.go
index 23989d3..b84bb6a 100644
--- a/cc/stl.go
+++ b/cc/stl.go
@@ -46,6 +46,8 @@
 				"stlport_shared", "stlport_static",
 				"gnustl_static":
 				return "ndk_lib" + stl.Properties.Stl
+			case "none":
+				return ""
 			default:
 				ctx.ModuleErrorf("stl: %q is not a supported STL with sdk_version set", stl.Properties.Stl)
 				return ""