[CodeGen] Include llvm/CodeGen/DFAPacketizer.h

This patch teaches MachinePipeliner.h to include
DFAPacketizer.h so that the header file is self contained.

Without this patch, a C++ source file that only includes
MachinePipeliner.h does not compile because the forward declaration of
DFAPacketizer is not enough for:

  llvm::SmallVector<std::unique_ptr<DFAPacketizer>> DFAResources;