Update kernel version to 5.18 in header constant tests

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

Tested with build-many-glibcs.py.