[compiler] Add include guard for code-assembler.h

Clients of the code-assembler.h shouldn't depend on lots of compiler
internals. Do not include anything from src/compiler there!

R=mstarzinger@chromium.org

Bug: v8:8834
Change-Id: Ie6f66af47153fad0b95c53d4d9cb03b95d3501fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1526196Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60286}

src/compiler/DEPS

+# Copyright 2019 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Clients of these interfaces shouldn't depend on lots of compiler internals.
+# Do not include anything from src/compiler here!
+specific_include_rules = {
+  "code-assembler.h": [
+    "-src/compiler",
+  ],
+  "pipeline.h": [
+    "-src/compiler",
+  ],
+  "wasm-compiler.h": [
+    "-src/compiler",
+  ],
+}