Deal with fallout from Bazel removing weird behavior for hidden attrs

Before, hidden attributes were implicitly looked up in the default
repository.

Change-Id: Iefb9fd24b73f8a0dfd7e488236fe53d26b0d8314
diff --git a/tools/build_rules/fortran.bzl b/tools/build_rules/fortran.bzl
index c67884a..22357e4 100644
--- a/tools/build_rules/fortran.bzl
+++ b/tools/build_rules/fortran.bzl
@@ -53,7 +53,7 @@
     # TODO(Brian): Replace this with something more fine-grained from the
     # configuration fragment or something.
     '_cc_toolchain': attr.label(
-      default = Label('//tools/cpp:toolchain'),
+      default = Label('@//tools/cpp:toolchain'),
     ),
   },
   outputs = _define_fortran_output,