Mark internal dirent functions hidden

Since internal dirent functions are only used internally in ld.so and
libc.so, they can be made hidden.

	[BZ #19122]
	* include/dirent.h (__opendirat): Add attribute_hidden.
	(__getdents): Likewise.
	(__getdents64): Likewise.
	(__alloc_dir): Likewise.
	[IS_IN (rtld)] (__closedir): Likewise.
	[IS_IN (rtld)] (__fdopendir): Likewise.
	[IS_IN (rtld)] (__readdir): Likewise.
	[IS_IN (rtld)] (__readdir64): Likewise.
	[IS_IN (rtld)] (__rewinddir): Likewise.