[DFAPacketizer] Move DefaultVLIWScheduler class declaration to header file

This change moves "DefaultVLIWScheduler" class declaration from
DFAPacketizer.cpp to DFAPacketizer.h.
This is needed because there is a protected class member of
type "DefaultVLIWScheduler*" in "VLIWPacketizerList" class.
The derived classes cannot use this memeber unless declaration
is available to it. More specifically :

// Without this change

```
class HexagonPacketizerList : public VLIWPacketizerList {
  public :
	HexagonPacketizerList() {
	// Below line will cause incomplete class error since
	// declaration was not available through header.
	VLIWScheduler->schedule();
  }
}
```

Reviewed By: kparzysz

Differential Revision: https://reviews.llvm.org/D139767