Update kernel version to 5.19 in header constant tests

[Florian Weimer]

* Joseph Myers:

> This patch updates the kernel version in the tests tst-mman-consts.py,
> tst-mount-consts.py and tst-pidfd-consts.py to 5.18.  (There are no
> new constants covered by these tests in 5.19, or in 5.17 or 5.18 in
> the case of tst-mount-consts.py that previously used version 5.16,
> that need any other header changes.)
>
> Tested with build-many-glibcs.py.

Looks okay, thanks.

Florian