config: Don't warn about missing conf.d directory

Hi! I noticed on NixOS I was seeing this warning:

Failed to open /etc/geoclue/conf.d/: Error opening directory “/etc/geoclue/conf.d/”: No such file or directory

The meson build installs this as an empty directory, but NixOS ignores this because of the way it generates configuration files.

It'd be possible to workaround the warning by automatically creating an empty directory, but distros that don't include any overrides shouldn't be forced to create an empty directory just to suppress the warning.