hurd: Fix `seekdir' symbol exposition from `rewinddir'

`seekdir' is MISC || XOPEN, it should not be exposed along `rewinddir' which
is POSIX.

	* include/dirent.h (__seekdir): New declaration.
	* sysdeps/mach/hurd/seekdir.c (seekdir): Rename to __seekdir and
	redefine as weak alias.
	* sysdeps/mach/hurd/rewinddir.c (__rewinddir): Use __seekdir instead
	of seekdir.