Fix build with CMake 3.10

Looks like a classic false-positive, but this makes the compile
pass for me without making the code harder to read:

AutoMoc error
-------------
  "/ssd/milian/projects/kf5/src/kde/workspace/kwin/kcmkwin/kwinscripts/main.cpp"
The file contains a K_PLUGIN_FACTORY macro, but does not include "main.moc"!
Consider to
 - add #include "main.moc"
 - enable SKIP_AUTOMOC for this file

So we just add the include and then get rid of the duplicate
definition of the plugin factory and the problem is resolved.