scripts/spdxcheck.py: Add dual license subdirectory

The licenses from the other directory were partially moved to the dual
directory in commit 8ea8814f ("LICENSES: Clearly mark dual license only
licenses"). checkpatch therefore rejected files like
drivers/staging/android/ashmem.h with

  WARNING: 'SPDX-License-Identifier: GPL-2.0 OR Apache-2.0 */' is not supported in LICENSES/...
  #1: FILE: drivers/staging/android/ashmem.h:1:
  +/* SPDX-License-Identifier: GPL-2.0 OR Apache-2.0 */

Fixes: 8ea8814f ("LICENSES: Clearly mark dual license only licenses")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/spdxcheck.py

@@ -32,7 +32,7 @@ class SPDXdata(object):
 def read_spdxdata(repo):
 
     # The subdirectories of LICENSES in the kernel source
-    license_dirs = [ "preferred", "deprecated", "exceptions" ]
+    license_dirs = [ "preferred", "deprecated", "exceptions", "dual" ]
     lictree = repo.head.commit.tree['LICENSES']
 
     spdx = SPDXdata()