botan: add type to BR2_PACKAGE_BOTAN_ARCH_SUPPORTS option

Commit 1737af36 ('botan: limit to
supported CPU architectures') introduced the
BR2_PACKAGE_BOTAN_ARCH_SUPPORTS with a default value, but forgot to
define a type, which leads to a kconfig warning:

  package/botan/Config.in:1:warning: config symbol defined without type

This commit fixes that by properly setting the type.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>