nx_sd: add sd_get_card_mounted() for screenshot and other callers

Made-with: Cursor

source/nx_sd.c

@@ -184,6 +184,11 @@ static void _sd_deinit()
 void sd_unmount() { _sd_deinit(); }
 void sd_end()     { _sd_deinit(); }
 
+bool sd_get_card_mounted(void)
+{
+	return sd_mounted;
+}
+
 void *sd_file_read(const char *path, u32 *fsize)
 {
 	FIL fp;