kernel: expose (unhide) CONFIG_ASN1 as ksmbd requirement

kernel: expose (unhide) CONFIG_ASN1 as ksmbd requirement

OpenWrt provides kmod-asn1-decoder for CONFIG_ASN1 but selecting it
doesn't really work as expected. Kernel symbol is hidden and can be
actually selected only as a dependency. That works well for in-kernel
stuff but fails for external modules requiring ASN1 like ksmbd.

Modify kernel Kconfig to make CONFIG_ASN1 always selectable. It's
required to satisfy ksmbd dependencies cleanly (without hack like
selecting unrelated modules).

Link: http://lists.openwrt.org/pipermail/openwrt-devel/2023-January/040298.html
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>