Add vendor

Specifically because I want to modify a dependency

vendor/golang.org/x/mobile/internal/mobileinit/mobileinit.go

@@ -0,0 +1,11 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package mobileinit contains common initialization logic for mobile platforms
+// that is relevant to both all-Go apps and gobind-based apps.
+//
+// Long-term, some code in this package should consider moving into Go stdlib.
+package mobileinit
+
+import "C"