Fix a lld-x86_64-darwin13 build error.

Summary:
Fix this build error

http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/15112/steps/build_Lld/logs/stdio

after https://reviews.llvm.org/rL318693

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40266

llvm-svn: 318696