script/ptl-tool: fix redmine_linkage_correct case in _get_active_issues()

If redmine_linkage_correct is True, _get_active_issues() is supposed to
ignore issues with "Multiple Source PRs" category.  However that doesn't
happen because the category string is compared against (category, text)
pair.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>