Make sure the dbus xml interface file exists before it's used

Summary:
This started to happen with cmake 3.9, producing this erorr:

• No rule to make target 'app/org.kde.khotkeys.xml', needed by

'kcm_hotkeys/khotkeys_interface.cpp'."

Test Plan: Builds fine with cmake 3.9.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D6792