Fix license for mockito

Running 'buck test' fails with:

  BUILD FAILED: Couldn't get dependency '//lib:LICENSE-MIT' of target '//plugins/multi-master:mockito':
  No rule found when resolving target //lib:LICENSE-MIT in build file //lib/BUCK

Change the license from 'MIT' to 'DO_NOT_DISTRIBUTE'.

Change-Id: Ie41494c60bc12cb9c151e2ebe14428fedfdc22ba
diff --git a/BUCK b/BUCK
index bc9d709..dcbf446 100644
--- a/BUCK
+++ b/BUCK
@@ -56,7 +56,7 @@
   name = 'mockito',
   id = 'org.mockito:mockito-core:2.5.0',
   sha1 = 'be28d46a52c7f2563580adeca350145e9ce916f8',
-  license = 'MIT',
+  license = 'DO_NOT_DISTRIBUTE',
   deps = [
     ':byte-buddy',
     ':objenesis',