management: add missing inline to wireEncode

Change-Id: I6d2eee4f2a04ccdf3842de3fe4f7ee52a8add3ac
diff --git a/src/management/nfd-face-event-notification.hpp b/src/management/nfd-face-event-notification.hpp
index 4a299d3..8e31103 100644
--- a/src/management/nfd-face-event-notification.hpp
+++ b/src/management/nfd-face-event-notification.hpp
@@ -112,7 +112,7 @@
   return totalLength;
 }
 
-const Block&
+inline const Block&
 FaceEventNotification::wireEncode() const
 {
   if (m_wire.hasWire ())