Properly find the epoxy config module
I made a mistake when adding this find_dependency call for epoxy. The find module for epoxy is actually lives in ECM (Extra CMake Modules.) So if a project that does find_package(KWin) does not use ECM, they are shown a really confusing error that epoxy could not be found.
To fix this, KWinConfig adds a find_dependency call for ECM explicitly.