Build singleusergroup with Buck

Change-Id: I21c1eb5e0737688e77fd82d51f6f2a469506c2a1

BUCK

diff --git a/BUCK b/BUCK
new file mode 100644
index 0000000..f1b0c92
--- /dev/null
+++ b/BUCK
@@ -0,0 +1,4 @@
+gerrit_plugin(
+  name = 'singleusergroup',
+  srcs = glob(['src/main/java/**/*.java']),
+)