libelf: Make elf_version thread-safe

	* elf_version.c (version_once): Define once.
	(initialize_version): New static function.
	(elf_version): Use initialize_version version_once.

Signed-off-by: Heather S. McIntyre <hsm2@rice.edu>
Signed-off-by: Mark Wielaard <mark@klomp.org>