Update kernel version to 5.7 in tst-mman-consts.py.

This patch updates the kernel version in the test tst-mman-consts.py
to 5.7.  (There are no new constants covered by this test in 5.7 that
need any other header changes; there's a new MREMAP_DONTUNMAP, but
this test doesn't yet cover MREMAP_*.)

Tested with build-many-glibcs.py.