STM32H7 - Fix building targets that can optionally define USE_SDCARD

i.e. MINIMALH750_EXST doesn't currently build, but should when
USE_SDCARD is enabled in the target config.

The issue arises from target .mk file's FEATURES list in combination
with STM32H7.mk including sdcard specific file when the SDCARD_SDIO
feature is enabled.

src/main/msc/usbd_storage_sdio.c

@@ -32,6 +32,8 @@
 
 #include "platform.h"
 
+#ifdef USE_SDCARD
+
 #include "common/utils.h"
 #include "drivers/dma.h"
 #include "drivers/dma_reqmap.h"
@@ -308,3 +310,4 @@ static int8_t STORAGE_GetMaxLun (void)
 
 /************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/
 
+#endif // USE_SDCARD