thread: Add a missing include of asan.h

I didn't notice this during testing because invariants-enabled kernels
implicitly include asan.h via kassert.h.

Reported by: Lexi Winter <lexi@le-Fay.org>
Fixes: 800da341bc4a ("thread: Simplify sanitizer integration with thread creation")

(cherry picked from commit 7a7063cc54274a44192fb65c71360a5e72c171b9)