linuxkpi: Add <linux/pgtable.h>

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 anot

linuxkpi: Add <linux/pgtable.h>

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.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38153

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