Update RequestHandler_SceneItems.cpp

In order to fully close the issue of #906 of obs-websockets, #include <optional> is necessary (v32 of OBS as of this commit).

Author: LinuxMainframe

src/requesthandler/RequestHandler_SceneItems.cpp

@@ -16,7 +16,7 @@ GNU General Public License for more details.
 You should have received a copy of the GNU General Public License along
 with this program. If not, see <https://www.gnu.org/licenses/>
 */
-
+#include <optional>
 #include "RequestHandler.h"
 
 /**