[ELF] - Attempt to fix ubuntu 64x buildbot (2).

Fixed inaccurate member type: uint32_t -> size_t
(http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/2984/steps/build/logs/stdio).

llvm-svn: 289796