veyron/profiles/gce: TBR: Add missing build tag.

Change-Id: I912fc5d54f5ca7195a8796467f159eb27e5c61cb
diff --git a/profiles/gce/initx.go b/profiles/gce/initx.go
index 839e270..2353eae 100644
--- a/profiles/gce/initx.go
+++ b/profiles/gce/initx.go
@@ -1,3 +1,7 @@
+// +build linux
+
+// Package gce provides a profile for Google Compute Engine and should be
+// used by binaries that only ever expect to be run on GCE.
 package gce
 
 import (