Use `inline` on all namespace-scope constants defined in headers

Change-Id: Ic61f8d26f59fdc2895415ca47065c02d6eec28ed
diff --git a/daemon/mgmt/rib-manager.cpp b/daemon/mgmt/rib-manager.cpp
index 24d52b7..afeca1c 100644
--- a/daemon/mgmt/rib-manager.cpp
+++ b/daemon/mgmt/rib-manager.cpp
@@ -44,7 +44,7 @@
 
 const std::string MGMT_MODULE_NAME = "rib";
 const Name LOCALHOST_TOP_PREFIX = "/localhost/nfd";
-const time::seconds ACTIVE_FACE_FETCH_INTERVAL = 5_min;
+constexpr time::seconds ACTIVE_FACE_FETCH_INTERVAL = 5_min;
 
 RibManager::RibManager(rib::Rib& rib, ndn::Face& face, ndn::KeyChain& keyChain,
                        ndn::nfd::Controller& nfdController, Dispatcher& dispatcher)