Move generic <bits/unistd_ext.h> to a more regular location
No functional change; the previous path worked as well, but it
re-added the obsolete sysdeps/generic/bits directory, which was
removed (for the first time) in commit
c72565e5.
Fixes commit e47d82c9 ("Provide
<bits/unistd_ext.h> as a sysdeps header exclusively").