scl/scl.conf: Use a wildcard to include SCL snippets

To be able to install SCL snippets to be included by default, without
modifying scl.conf itself, use a wildcard-include. This allows
distributions to ship some of the SCLs in different packages, without
breaking the default config, or having to enable them in other ways.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>