Generate XDK headers at build time

wdm.h, ntddk.h, ntifs.h, devioctl.h, ntdef.h and winnt.h are auto-generated from XDK headers.
This means to properly update such a header you have to change the XDK version, then run hpp by hand. The build system can do this automatically, so it should! The generated files should then be removed from source control.