[AArch64] Fix -Wunused-variable in AArch64FrameLowering.cpp (NFC)

llvm-project/llvm/lib/Target/AArch64/AArch64FrameLowering.cpp:497:21:
error: unused variable 'MFI' [-Werror,-Wunused-variable]
  MachineFrameInfo &MFI = MF.getFrameInfo();
                    ^
1 error generated.