Add an empty tools/BUILD file explicitly for Bazel

Change-Id: I883980556f5acdafb2f8a8b63bb8ad5893fa7a0c
diff --git a/tools/BUILD b/tools/BUILD
new file mode 100644
index 0000000..c5ed0b7
--- /dev/null
+++ b/tools/BUILD
@@ -0,0 +1 @@
+# Empty file required by Bazel