Ignore MAP_VARIABLE in tst-mman-consts.py

Linux 6.2 removed the hppa compatibility MAP_VARIABLE define.  That
means that, whether or not we remove it in glibc, it needs to be
ignored in tst-mman-consts.py (since this macro comparison
infrastructure expects that new kernel header versions only add new
macros, not remove old ones).

Tested with build-many-glibcs.py for hppa-linux-gnu (Linux 6.2
headers).