Fix crash when dropping effect on a group

[kdenlive] / src / effectstack / CMakeLists.txt

set(kdenlive_SRCS
  ${kdenlive_SRCS}
  effectstack/abstractcollapsiblewidget.cpp
  effectstack/collapsibleeffect.cpp
  effectstack/collapsiblegroup.cpp
  effectstack/effectstackview2.cpp
  PARENT_SCOPE
)