misc: fs_loader: Fix compile warnings when CONFIG_CMD_UBIFS is enabled

drivers/misc/fs_loader.c: In function ‘mount_ubifs’:
drivers/misc/fs_loader.c:46:12: warning: implicit declaration of function ‘ubi_part’ [-Wimplicit-function-declaration]
  int ret = ubi_part(mtdpart, NULL);
            ^~~~~~~~
drivers/misc/fs_loader.c:53:9: warning: implicit declaration of function ‘cmd_ubifs_mount’ [-Wimplicit-function-declaration]
  return cmd_ubifs_mount(ubivol);
         ^~~~~~~~~~~~~~~
drivers/misc/fs_loader.c: In function ‘umount_ubifs’:
drivers/misc/fs_loader.c:58:9: warning: implicit declaration of function ‘cmd_ubifs_umount’ [-Wimplicit-function-declaration]
  return cmd_ubifs_umount();
         ^~~~~~~~~~~~~~~~

Signed-off-by: Pali Rohár <pali@kernel.org>