Add an empty tools/BUILD file explicitly for Bazel

To make sure that all plugins consistently have empty BUILD files
within their tools.

Change-Id: I2b5f59e27e2ee8813116f61a941b28061fcc3b25

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
new file mode 100644
index 0000000..1fa2160
--- /dev/null
+++ b/tools/BUILD
@@ -0,0 +1 @@
+# Empty file - bazel treat directories with BUILD file as a package
\ No newline at end of file