Fix missing U8glib.h include (#14966)

Marlin/src/HAL/HAL_DUE/dogm/u8g_com_HAL_DUE_sw_spi_shared.h

 
 #include "../../../inc/MarlinConfigPre.h"
 #include "../../shared/Marduino.h"
+#include <U8glib.h>
 
 void u8g_SetPIOutput_DUE(u8g_t *u8g, uint8_t pin_index);
 void u8g_SetPILevel_DUE(u8g_t *u8g, uint8_t pin_index, uint8_t level);