Don't issue an error if DT_PLTRELSZ is missing

A shared object doesn't need PLT if there are no PLT relocations.  It
shouldn't be an error if DT_PLTRELSZ is missing.

	[BZ #18410]
	* elf/dl-reloc.c (_dl_relocate_object): Don't issue an error
	for missing DT_PLTRELSZ.