kwinscreenedges: Fix build (missing main.ui in kcm_kwintouchscreenedges_PART_SRCS)

Summary: kwinscreenedgeconfigform.cpp:23:10: fatal error: ui_main.h: No such file or directory

Test Plan: kwin builds again.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29212