stm32/boards/STM32F769DISC: Fix building with USE_QSPI_XIP=1.

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/boards/STM32F769DISC/board_init.c

@@ -3,13 +3,17 @@
 
 // This configuration is needed for mboot to be able to write to the external QSPI flash
 
+#if MICROPY_HW_SPIFLASH_ENABLE_CACHE
 STATIC mp_spiflash_cache_t spi_bdev_cache;
+#endif
 
 const mp_spiflash_config_t spiflash_config = {
     .bus_kind = MP_SPIFLASH_BUS_QSPI,
     .bus.u_qspi.data = NULL,
     .bus.u_qspi.proto = &qspi_proto,
+    #if MICROPY_HW_SPIFLASH_ENABLE_CACHE
     .cache = &spi_bdev_cache,
+    #endif
 };
 
 spi_bdev_t spi_bdev;