Add fltkernel.h to base headers

[wedsonaf]

This allows rust drivers to use fltmgr.

[wmmc88]

It looks like the change is causing redefinitions of the GUIDs when other cargo features are enabled (ex. HID + STORAGE). this seems to be caused by fltkernel.h adding initguid.h which is now causing all the other guids in other headers to be both declared and (re-)defined.

In bindgen_header_contents, you can special-case the ApiSubset case that includes fltkernel.h to also prepend something like:

// Force DEFINE_GUID to only declare GUIDS, and not define them (regardless if downstream headers try to include initguid.h)
#include <initguid.h>
#undef INITGUID
#include <guiddef.h>

[wmmc88]

Another thing of note is that fltkernel.h seems to be part of the Filesystem subset of the WDK. I think it makes sense to separate it out as its own cargo feature, similar to what was done in #260, #263, #278, #287, etc.