Export path of kwalletd binary for kwallet_pam

Summary:
kwallet_pam requires the path to the kwalletd binary. Currently this is
hardcoded.

Test Plan: Looked at generated file

Reviewers: apol

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20216