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.

kcmkwin/kwinscripts/main.cpp

@@ -22,3 +22,5 @@
 
 K_PLUGIN_FACTORY(KcmKWinScriptsFactory,
                  registerPlugin<Module>("kwin-scripts");)
+
+#include "main.moc"

kcmkwin/kwinscripts/module.cpp

@@ -40,8 +40,6 @@
 
 #include "version.h"
 
-K_PLUGIN_FACTORY_DECLARATION(KcmKWinScriptsFactory)
-
 Module::Module(QWidget *parent, const QVariantList &args) :
     KCModule(parent, args),
     ui(new Ui::Module),