scl/scl.conf: Do not force "plugin.conf" when including

Use a less strict wildcard when including other SCL snippets, otherwise
some of the existing plugins will not be loaded by default. (This also
brings 3.6's scl.conf in line with 3.7's.)

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #358 from algernon/h/3.6/scl-wildcard

scl/scl.conf: Do not force "plugin.conf" when including