slimserver: add perlPackages.NetHTTPSNB as a dependency

Slimserver v. 7.9.2 introduced the CPAN package `Net::HTTPS::NB`[1][2]
as a dependency (distributed directly inside the Git repository, cf. [3]).

Without this package present in the Perl include path, the new version
fails to load the repository database for squeezebox server plugins
(configured via `Settings > Plugins`) and thus does not list any plugins
that aren't already installed on the respective system.

[1]: https://metacpan.org/pod/Net::HTTPS::NB
[2]: https://github.com/olegwtf/p5-Net-HTTPS-NB
[3]: 7938c06119/CPAN/Net/HTTPS/NB.pm

pkgs/servers/slimserver/default.nix

@@ -44,6 +44,7 @@ perlPackages.buildPerlPackage rec {
     perlPackages.LogLog4perl
     perlPackages.LWP
     perlPackages.NetHTTP
+    perlPackages.NetHTTPSNB
     perlPackages.ProcBackground
     perlPackages.SubName
     perlPackages.TemplateToolkit