linuxkpi: Add <linux/pgtable.h>

[dumbbell]

Summary

For now, it's empty. It is included by the DRM drivers but nothing is missing otherwise. Perhaps something we already defined should be in pgtable.h instead of another header.

This is part of the update of the DRM drivers to Linux 5.14.

[manu]

This should go to the dummy directory.

[dumbbell]

Address @manu comment and add header under dummy.