GN: Add exception for //sandbox/policy.

TBR=dpranke@google.com

NOTRY=true

Bug: 896680
Change-Id: Id4a14d384bdff3dde45ce293a875733d7d97608f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288383
Commit-Queue: Erik Staab <estaab@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786418}

.gn

@@ -219,6 +219,7 @@ no_check_targets = [
   "//remoting:*",  # 27 errors
   "//sandbox/linux:*",  # 13 errors
   "//sandbox/mac:*",  # 14 errors
+  "//sandbox/policy:*",  # 1 error
   "//sandbox/win:*",  # 7 errors
   "//sandbox:*",  # 3 errors