mac: Allow reading all subdirs of /System/Library/KerberosPlugins.

Previously only KerberosFrameworkPlugins was allowed, but user reports
indicate needing to access a "GSSAPI" subdir as well.

Bug: 1134449
Change-Id: I298fd03e78457608f23700380a29ce225830f42e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518121Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823721}

sandbox/policy/mac/network.sb

@@ -120,5 +120,5 @@
 )
 (allow file-read*
   (path "/private/etc/krb5.conf")
-  (subpath "/System/Library/KerberosPlugins/KerberosFrameworkPlugins")
+  (subpath "/System/Library/KerberosPlugins") ; https://crbug.com/1134449
 )