[GWP-ASan] Fix thread ID.

Summary:
GWP-ASan currently reports <unknown> thread ID, as the crash handler
merge dropped the include. Oops.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: eugenis, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75693