meson: align default distconfdir with autotools

meson: align default distconfdir with autotools

Autotools uses $prefix/lib as the default for distconfdir, while meson
$libdir - which is not guaranteed to be the same.

In particular: Debian and derivatives use /usr/lib/$target-triplet as a
$libdir, while still keep the configuration directory as /usr/lib.

Fixes: 370141c1 ("meson: introduce meson, covering libkmod.so")
Reported-by: Marco d'Itri <md@linux.it>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/216
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>