Export LICENSE

This makes it easier to include the license in redistributions, as required by the license.
diff --git a/BUILD.bazel b/BUILD.bazel
index 7e227aa..8099642 100644
--- a/BUILD.bazel
+++ b/BUILD.bazel
@@ -36,6 +36,8 @@
 
 licenses(["notice"])
 
+exports_files(["LICENSE"])
+
 config_setting(
     name = "windows",
     constraint_values = ["@bazel_tools//platforms:windows"],