[roll] Roll fuchsia [kernel] Add Reserve method to KTrace

This CL adds a Reserve method to KTrace that wraps the pre-existing
internal::KTraceState::Reserve method. This means KTrace now
implements the fxt::Writer protocol, and the static instance can
now be passed in to the Emit* functions directly.

This is a functional no-op, but will make it easier to add support
for per-CPU buffers in the future.

Original-Bug: 404539312
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1243284
Original-Revision: 94ae7e14929e8382b757e5969079aa76a937b9ef
GitOrigin-RevId: dfff6e76a23d1ae20b104bafa0daefef64f85030
Change-Id: I416dce5f7afa54d30a7f642df210965f2d4821d5
diff --git a/stem b/stem
index b47c751..2350fe6 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="486f7b804ecbc58380fd6375bb688474a5d8ce8b"
+             revision="94ae7e14929e8382b757e5969079aa76a937b9ef"
              />
   </projects>
   <hooks>