readelf: Pass around GElf_Ehdr instead of calling gelf_getehdr

readelf: Pass around GElf_Ehdr instead of calling gelf_getehdr

handle_core_item calls gelf_getehdr for each item without checking if
the call succeeds. It should always succeed because process_elf_file
already checked gelf_getehdr returned a valid Ehdr before passing it
to handle_notes. Just pass the Ehdr down a couple more function calls.

	* readelf.c (handle_core_item): Take const Gelf_Ehdr and use it.
	(handle_core_items): Take const Gelf_Ehdr and pass it to
	handle_core_item.
	(handle_core_note): Take const Gelf_Ehdr and pass it to
	handle_core_items.
	(handle_notes_data): Pass ehdr to handle_core_note.

Signed-off-by: Mark Wielaard <mark@klomp.org>