Fix creation of kdeglobals if /etc/xdg/kdeglobals present

Summary:
If /etc/xdg/kdeglobals exists but ~/.config/kdeglobals
does not (like on the first start of kwin_wayland), it did not notice
changes to kdeglobals.
This lead to KWin using the wrong color scheme for the window decoration
on the first login, which also could not be changed without a session restart.

Test Plan: Cleared ~ and logged in again, title bar color is now correct.

Reviewers: #plasma, romangg

Reviewed By: #plasma, romangg

Subscribers: sitter, broulik, rikmills, kwin, plasma-devel

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D22238