update_engine: fix missing include lint error

BUG=None
TEST=unit tests

Change-Id: Idb4b28a3f785d64c520fc07b855f57d7618e8923
Reviewed-on: https://chromium-review.googlesource.com/882309
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/connection_manager.cc b/connection_manager.cc
index 5085842..c225cb2 100644
--- a/connection_manager.cc
+++ b/connection_manager.cc
@@ -16,6 +16,7 @@
 
 #include "update_engine/connection_manager.h"
 
+#include <memory>
 #include <set>
 #include <string>