kernel: add missing symbol (BLOCK_LEGACY_AUTOLOAD)

kernel: add missing symbol (BLOCK_LEGACY_AUTOLOAD)

As of kernel v5.15.179 kernel configuration fails without a
reference to BLOCK_LEGACY_AUTOLOAD, apparently as a consequence
of commit 3e9899c12d5ab3a8ce376bf4365175ea3dbaffc0 (backport of
6c0f5898836c05c6d850a750ed7940ba29e4e6c5), so add this symbol as
unset which is the same state the symbol has in the 6.6 kernel
config for 24.10.

Fixes: dfe1b5e82a35 ("kernel: Update to version 5.15.179")
Signed-off-by: Andrew MacIntyre <andymac@pcug.org.au>
Link: https://github.com/openwrt/openwrt/pull/18552
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>