cmake fixes for external OndselSolver building (#18379)

* Assembly: fix building with external OndselSolver

This commit pairs with f35b075e271e9cb3451f3439f7dc0274841a0838.
It solves two issues:

- remove hardcoded includes of 3rdparty files
- add missing include_directories()

* Assembly: add check for external OndselSolver includes

src/Mod/Assembly/App/AssemblyObject.h

@@ -33,7 +33,7 @@
 #include <App/PropertyLinks.h>
 #include "SimulationGroup.h"
 
-#include <3rdParty/OndselSolver/OndselSolver/enum.h>
+#include <OndselSolver/enum.h>
 
 namespace MbD
 {