Add missing acquire_load to call_once overload.

Summary: Seemed to have been overlooked in D24028.

This bug was found and brought to my attention by Paul Wankadia.

Reviewers: kubamracek, EricWF, dvyukov

Differential Revision: https://reviews.llvm.org/D32402

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@301132 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed