[OpenMP] Remove extra warning when we build

Summary:
This one line change is to remove this warning message

"warning: integer conversion resulted in a change of sign"

Reviewers: grokos

Reviewed By: grokos

Subscribers: openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D45415

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@329713 91177308-0d34-0410-b5e6-96231b3b80d8