Update our protobuf BUILD hack for newer Bazel

I think this might only matter for the version I'm running locally and
not the one on robotics.mvla.net.

Change-Id: I00f8d8eb5a3d61fc76c51a3d94fc28d02a64e5b8
diff --git a/third_party/protobuf/protobuf.bzl b/third_party/protobuf/protobuf.bzl
index 0cc195b..d91cde4 100644
--- a/third_party/protobuf/protobuf.bzl
+++ b/third_party/protobuf/protobuf.bzl
@@ -1,13 +1,13 @@
 # -*- mode: python; -*- PYTHON-PREPROCESSING-REQUIRED
 
 def _GetPath(ctx, path):
-  if str(ctx.label).startswith('@'):
+  if str(ctx.label).startswith('@') and not str(ctx.label).startswith('@//'):
     fail('External labels not supported for now')
   return path
 
 def _GenDir(ctx):
   if not ctx.attr.includes:
-    if str(ctx.label).startswith('@'):
+    if str(ctx.label).startswith('@') and not str(ctx.label).startswith('@//'):
       fail('External labels not supported for now')
     return ''
   if not ctx.attr.includes[0]: