Add `exports_files`

The LICENSE file wasn't exported.
diff --git a/BUILD b/BUILD
index d0e0108..6cab225 100644
--- a/BUILD
+++ b/BUILD
@@ -2,6 +2,8 @@
 
 licenses(["notice"])
 
+exports_files(["LICENSE"])
+
 cc_library(
     name = "double-conversion",
     srcs = [