Fix wrong iterator type

ELFEntityIterator does not implement RandomAccessIterator. It does
not even implement BidirectionalIterator.

This patch fixes LLD build issue when compiled with MSVC2013 with
debug: MSVC's find_if checks if the start iterator is before the end
iterator in the sense of operator< if it declares implementing
RandomAccessIterator. If a class does not have operator<, it fails
to compile.