relocate-rela: replace a magic number with sizeof(Elf64_Rela)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Scott Wood <scottwood@freescale.com>
Cc: David Feng <fenghua@phytium.com.cn>
Acked-by: Scott Wood <scottwood@freescale.com>