Move hidden_proto (__dl_iterate_phdr) to include/link.h [BZ #18822]

__dl_iterate_phdr is hidden and should be accessed directly within
libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* elf/dl-iteratephdr.c (hidden_proto (__dl_iterate_phdr)): Moved
	to ...
	* include/link.h (hidden_proto (__dl_iterate_phdr)): Here.