Make nbdkit-plugin-tar require nbdkit-plugin-perl.

The tar plugin is written in Perl and so won't work unless the Perl
plugin is also installed.

This dependency should be generated automatically, but for now we add
it manually.

(cherry picked from commit eb867f6953)

nbdkit.spec

@@ -344,6 +344,8 @@ Summary:        Tar archive plugin for %{name}
 License:        BSD
 
 Requires:       %{name}%{?_isa} = %{version}-%{release}
+# XXX This dependency should be autogenerated.
+Requires:       %{name}-plugin-perl
 
 
 %description plugin-tar