[lsan] Use final on Fuchsia ThreadContext declaration

This is consistent with other platforms' versions and
eliminates a compiler warning.

Reviewed By: leonardchan

Differential Revision: https://reviews.llvm.org/D92442

Author: frobtech

compiler-rt/lib/lsan/lsan_fuchsia.h

@@ -23,7 +23,7 @@
 
 namespace __lsan {
 
-class ThreadContext : public ThreadContextLsanBase {
+class ThreadContext final : public ThreadContextLsanBase {
  public:
   explicit ThreadContext(int tid);
   void OnCreated(void *arg) override;