Including header file for RtlRandomEx

Hi Guys,
I am hitting one problem. I need to use the random number generator function RtlRandomEx so I include the file “ntifs.h”. I start getting a lot of errors with this. In my global include file, this is the sequence of including the header files:-

#include <ntddk.h>

#pragma warning(disable:4201)  // nameless struct/union warning
#include <stdarg.h>
#include <stddef.h>
#include <wdf.h>

#pragma warning(default:4201)

#include <initguid.h>
#include <wdmguid.h>

To avoid the errors I looked up the declaration for the function and stick it in one of my header files. This works, but I know I am not doing something right.

How can I get the declaration of RtlRandomEx in my driver without doing something fishy?

Thanks
-AJ