sw64: enable CONFIG_IKHEADERS in defconfig

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



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Add CONFIG_IKHEADERS=y which enables access to the in-kernel headers to
defconfig.

Signed-off-by: Mao Minkai <maominkai@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>