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

Change-Id: Ic61f8d26f59fdc2895415ca47065c02d6eec28ed
diff --git a/daemon/table/pit-face-record.cpp b/daemon/table/pit-face-record.cpp
index c0f43bc..4b96c40 100644
--- a/daemon/table/pit-face-record.cpp
+++ b/daemon/table/pit-face-record.cpp
@@ -28,7 +28,7 @@
 namespace nfd::pit {
 
 // Impose a maximum lifetime to prevent integer overflow when calculating m_expiry.
-const time::milliseconds MAX_LIFETIME = 10_days;
+constexpr time::milliseconds MAX_LIFETIME = 10_days;
 
 void
 FaceRecord::update(const Interest& interest)