Changes in header file not detected

Header files may erroneously be disconnected from the dependency list of CPP object files if they are also used as input for a rule in another product.  This can happen if changes are made to the other project causing it to be reparsed. The problem can be reproduced in the attached example by running:

qbs
sed -i 's/cat/echo/g' lib2/lib2.qbs

You can now touch `lib1/lib1.h` and build again, but `app.cpp` which indirectly includes `lib1.h` won't get recompiled as it should. `lib1.h` has been removed from the `childrenAddedByScanner` list in `app.cpp.o`'s artifact.