installdbgsymbols_suse: Pass --plus-content debug to zypper

Summary:
This way the repository with debuginfo does not have to be enabled
manually before clicking on the button.
This option works again now, so it should be passed as default.

BUG: 416038

Test Plan: Works locally. Reported success on the linked bug.

Reviewers: lbeltrame, cgiboudeaux

Reviewed By: cgiboudeaux

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24751