move ECDH_CTX into cpp

Change-Id: Ib28e0d0586f9e89fe2aaaaa3c6311ffd0325f8ad
diff --git a/src/requester.cpp b/src/requester.cpp
index de2d1a6..c7717a7 100644
--- a/src/requester.cpp
+++ b/src/requester.cpp
@@ -202,7 +202,7 @@
 
   // ECDH and HKDF
   state.m_ecdh.deriveSecret(content.ecdhKey);
-  hkdf(state.m_ecdh.context->sharedSecret, state.m_ecdh.context->sharedSecretLen,
+  hkdf(state.m_ecdh.m_sharedSecret, state.m_ecdh.m_sharedSecretLen,
        (uint8_t*)&content.salt, sizeof(content.salt), state.m_aesKey, sizeof(state.m_aesKey));
 
   // update state