[mlir] Fix -Wunused-variable in ROCDL/Target.cpp (NFC)

/llvm-project/mlir/lib/Target/LLVM/ROCDL/Target.cpp:181:40: error: unused variable 'targetMachine' [-Werror,-Wunused-variable]
  std::optional<llvm::TargetMachine *> targetMachine =
                                       ^
1 error generated.