Fix compile error when backlight support is enabled

Author: thomasjfox

Arduino_Pedelec_Controller/display.cpp

@@ -17,6 +17,7 @@ along with this program; if not, write to the Free Software Foundation,
 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301  USA
 */
 #include "display.h"
+#include "display_backlight.h"
 #include "config.h"