docs: Fix doxgyen build

Change-Id: I545ef94f5076592e605b08e906d90d7857947fa8
diff --git a/src/state.cpp b/src/state.cpp
index 0b2c0c7..26a8944 100644
--- a/src/state.cpp
+++ b/src/state.cpp
@@ -117,9 +117,11 @@
   return totalLength;
 }
 
+//! \relates State
 template size_t
 State::wireEncode<true>(ndn::EncodingImpl<true>& block) const;
 
+//! \relates State
 template size_t
 State::wireEncode<false>(ndn::EncodingImpl<false>& block) const;