Update visibility of files (#243)

Stardoc needs access to a file (with exports_files) in order to
generate its documentation.
diff --git a/BUILD b/BUILD
index 9999fb5..c0ccdcd 100644
--- a/BUILD
+++ b/BUILD
@@ -4,10 +4,7 @@
 
 package(default_visibility = ["//visibility:public"])
 
-exports_files([
-    "LICENSE",
-    "lib.bzl",
-])
+exports_files(["LICENSE"])
 
 filegroup(
     name = "test_deps",