build: switch to C++17

Change-Id: Ie68020a04b7e20b74778b6d0370544ded55c5e26
diff --git a/tests/daemon/face/lp-reliability.t.cpp b/tests/daemon/face/lp-reliability.t.cpp
index e3a84c9..526708f 100644
--- a/tests/daemon/face/lp-reliability.t.cpp
+++ b/tests/daemon/face/lp-reliability.t.cpp
@@ -33,6 +33,7 @@
 #include "dummy-transport.hpp"
 
 #include <cstring>
+#include <unordered_set>
 
 namespace nfd {
 namespace face {