storage: add sd_get_card_mounted declaration

bdk/storage/nx_sd.h

@@ -46,6 +46,7 @@ void sd_error_count_increment(u8 type);
 u16 *sd_get_error_count();
 bool sd_get_card_removed();
 bool sd_get_card_initialized();
+bool sd_get_card_mounted();
 u32  sd_get_mode();
 int  sd_init_retry(bool power_cycle);
 bool sd_initialize(bool power_cycle);