add namespace for requester

Change-Id: I3de037dffd702edff9f87a87e8e684161db3d329
diff --git a/src/detail/challenge-encoder.cpp b/src/detail/challenge-encoder.cpp
index f3ebb5a..fed4110 100644
--- a/src/detail/challenge-encoder.cpp
+++ b/src/detail/challenge-encoder.cpp
@@ -40,7 +40,7 @@
 }
 
 void
-ChallengeEncoder::decodeDataContent(const Block& data, RequesterState& state)
+ChallengeEncoder::decodeDataContent(const Block& data, requester::RequesterState& state)
 {
   data.parse();
   state.m_status = static_cast<Status>(readNonNegativeInteger(data.get(tlv::Status)));