This fixes the following compile warning:
CC init/do_mounts.o
init/do_mounts.c:478:19: warning: 'mount_ubi_rootfs' defined but not used [-Wunused-function]
478 | static int __init mount_ubi_rootfs(void)
| ^~~~~~~~~~~~~~~~
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>