Fix warnings when -Wextra-semi is enabled

Also, add argument -Wextra-semi if supported by the compiler.

dbsim/db_server.hpp

@@ -77,6 +77,6 @@ namespace db
         std::vector<std::shared_ptr<db::client>> clients_;
         std::vector<boost::thread> client_threads_;
     };
-};
+}
 
 #endif // WSREP_DB_SERVER_HPP

dbsim/db_tls.hpp

@@ -57,6 +57,6 @@ namespace db
         static void init(int mode);
         static std::string stats();
     };
-};
+}
 
 #endif // WSREP_DB_TLS_HPP