Don't build pci module on ARM

Summary:
so far all (supported by us, at least somewhat)
arm systems don't have pci at all, so this module
beside crashing doesn't do anything at all

BUG:335908

Test Plan: no kcm at all on arm

Reviewers: #plasma

Reviewed By: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9682