libtasn1: Disable code not needed in GRUB

libtasn1: Disable code not needed in GRUB

We don't expect to be able to write ASN.1, only read it,
so we can disable some code.

Do that with #if 0/#endif, rather than deletion. This means
that the difference between upstream and GRUB is smaller,
which should make updating libtasn1 easier in the future.

With these exclusions we also avoid the need for minmax.h,
which is convenient because it means we don't have to
import it from gnulib.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>