routing: use unique_ptr for signal container

Change-Id: I1912f8630d0e26d8c25c2050208f6b8b0ffd9009
refs: #4268
diff --git a/src/route/name-prefix-table.hpp b/src/route/name-prefix-table.hpp
index f4901ba..1e11e15 100644
--- a/src/route/name-prefix-table.hpp
+++ b/src/route/name-prefix-table.hpp
@@ -41,7 +41,7 @@
   using NptEntryList = std::list<std::shared_ptr<NamePrefixTableEntry>>;
   using const_iterator = NptEntryList::const_iterator;
 
-  NamePrefixTable(Nlsr& nlsr, std::shared_ptr<AfterRoutingChange>& afterRoutingChangeSignal);
+  NamePrefixTable(Nlsr& nlsr, std::unique_ptr<AfterRoutingChange>& afterRoutingChangeSignal);
 
   ~NamePrefixTable();