Add missing include in simulator-base.h

Fixes ppc and s390:

https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20ppc%20-%20sim/builds/10144
https://build.chromium.org/p/client.v8.ports/builders/V8%20Linux%20-%20s390%20-%20sim/builds/10698

TBR=yangguo@chromium.org

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6666
Change-Id: I1134218d04f328980e97d1fe1f0d0e7d7d779a1d
Reviewed-on: https://chromium-review.googlesource.com/973611Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52117}

src/simulator-base.h

@@ -9,6 +9,7 @@
 
 #include "src/assembler.h"
 #include "src/globals.h"
+#include "src/isolate.h"
 
 #if defined(USE_SIMULATOR)