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
c72565e5f1124c2dc72573e83406fe999e56091f.
Fixes commit e47d82c99a6db060419b421768aced76bea92997 ("Provide
<bits/unistd_ext.h> as a sysdeps header exclusively").
