[profile] gcov_mutex must be static

Summary: Forget static keyword for gcov_mutex in https://reviews.llvm.org/D74953 and that causes test failure on mac.

Reviewers: erik.pilkington, vsk

Reviewed By: vsk

Subscribers: vsk, dexonsmith, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75080