[roll] Roll fuchsia [clang] Add missing include

Add include file `cstdarg` which was transitively added with the current
toolchain revision but will fail compilation with newer toolchain
revisions.

Original-Bug: 389967864
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1188152
Original-Revision: b63cee3d18d0193faae7a3cf9ae449e203ed2af1
GitOrigin-RevId: b08874d0bcdda4bce466992e01bc4518a2c25a4b
Change-Id: I9c6387068b0e3bde55c28116051b2222c334e693
diff --git a/stem b/stem
index a377caa..af52a9f 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="56217fb06dd305b4dd38c9c52dc9b46c32812cca"/>
+             revision="b63cee3d18d0193faae7a3cf9ae449e203ed2af1"/>
   </projects>
   <hooks>
     <hook name="install-environment"