fix undefined constexpr symbols on osx

static constexpr in class/struct need to be redeclared in namespace
scope until C++17 (at which point it will be deprecated).

Change-Id: I27f0448edd65f9d7d51ad5eb7263c76d17563072
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

src/peer_connection.cpp

@@ -77,6 +77,9 @@ public:
     std::promise<bool> connected;
 };
 
+// Declaration at namespace scope is necessary (until C++17)
+constexpr std::chrono::seconds TlsTurnEndpoint::Impl::TLS_TIMEOUT;
+
 TlsTurnEndpoint::Impl::~Impl()
 {
     if (peerCertificate)
@@ -285,6 +288,9 @@ public:
     const dht::crypto::Certificate& peerCertificate;
 };
 
+// Declaration at namespace scope is necessary (until C++17)
+constexpr std::chrono::seconds TlsSocketEndpoint::Impl::TLS_TIMEOUT;
+
 int
 TlsSocketEndpoint::Impl::verifyCertificate(gnutls_session_t session)
 {