tls_session: fix build warning

"override" keyword missing for a method in TlsSession class declaration.

Change-Id: Ie8e98da6bd9a20246464593460b058200ee1c08b
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

src/security/tls_session.h

@@ -114,7 +114,7 @@ public:
 
     /// Request TLS thread to stop and quit.
     /// \note IO operations return error after this call.
-    void shutdown();
+    void shutdown() override;
 
     void setOnRecv(RecvCb&& cb) override {
         (void)cb;