qpainterpath.h and qtransform.h

This is an follow up to QTBUG-82602

In qtransform.h the include for qpainterpath.h has been replaced by a forward declaration. Because of that the code of Qwt 6.1 does not compile anymore.

Adding the missing include to the qwt code base can of course be done easily and having this include there would have been the correct implementation.

But Qwt 6.1 is used in trillions of projects and is part of every major linux distri.
If you do not want create an update scenario for the maintainer ( me ) and all Qwt users the missing include has to be reintroduced to qtransform.h

The problem has been introduced with Beta2 and did not exist with Beta1.