How to using PcdGetEx to change a PCD.

[krishnaLee <sssky307@163.com>]

Hello,
I am learning and writing a application to change the Nt32pkg-virtual machine's boot time.follow code is success.
but I think if I using PcdGetEx16 function, it will also work well,but failed when I compile it.
I had read some Specs about the difference between PcdGetEx and PcdGet,but I can't understand,maybe I need some practice,
So can anyone modify it to using PcdGetEx to implement the same function in the application?
//source-code--start
EFI_STATUS
EFIAPI
UefiMain (
IN EFI_HANDLE ImageHandle,
IN EFI_SYSTEM_TABLE *SystemTable
)
{
UINTN buffer=0;
UINTN index;
buffer=PcdGet16(PcdPlatformBootTimeOut);
//buffer=PcdGetEx16(&gEfiMdePkgTokenSpaceGuid,PcdPlatformBootTimeOut);//compile failed.
Print(L"buffer:%d\n",buffer);
PcdSet16(PcdPlatformBootTimeOut,5);
...


//source-code-end.


attachment is the full source code.
edk2-vUDK2017's build command:
build -p Nt32Pkg\Nt32Pkg.dsc -m Nt32Pkg\Application\mytestpcd2\mytestpcd2.inf


thank you very much!