Don't hardcode default kwallet path

Summary:
This allows a user to install to a different prefix and have kwallet_pam
work out the box.

Test Plan: My kwallet_pam now works

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20218