shorewall6 : add dependency on perl-socket6

Starting shorewall6 on a fresh install gives the following error message.

Can't locate Socket6.pm in @INC (you may need to install the Socket6 module) (@INC contains: /etc/shorewall6/ /usr/share/shorewall6/ /usr/share/shorewall/ /usr/share/shorewall6/deprecated/ /usr/share/shorewall/deprecated/ /usr/share/shorewall /usr/local/lib/perl5/site_perl /usr/local/share/perl5/site_perl /usr/lib/perl5/vendor_perl /usr/share/perl5/vendor_perl /usr/lib/perl5/core_perl /usr/share/perl5/core_perl) at /usr/share/shorewall/Shorewall/IPAddrs.pm line 511, <$currentfile> line 9.                                                                                                                 [ !! ]
 * ERROR: shorewall6 failed to start

This can be fixed by manually adding the perl-socket6 package. Therefore suggest it is added to shorewall6 dependencies to be installed automatically.

(from redmine: issue id 10158, created on 2019-03-26)