[BSP] fix compiling issue

bsp/efm32/graphics/tftspi.h

@@ -16,6 +16,8 @@
 #ifndef __TFTSPI_H__
 #define __TFTSPI_H__
 
+#include <rtthread.h>
+
 /* Includes ------------------------------------------------------------------*/
 /* Exported types ------------------------------------------------------------*/
 /* Exported constants --------------------------------------------------------*/