export,module: add SPDX GPL-2.0 license identifier to headers with no license

Commit b2441318 ("License cleanup: add SPDX GPL-2.0 license
identifier to files with no license") took care of a lot of files
without any license information.

These headers were not processed by the tool perhaps because they
contain "GPL" in the code.

I do not see any license boilerplate in them, so they fall back to
GPL version 2 only, which is the project default.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20191018045053.8424-1-yamada.masahiro@socionext.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/asm-generic/export.h

+/* SPDX-License-Identifier: GPL-2.0-only */
 #ifndef __ASM_GENERIC_EXPORT_H
 #define __ASM_GENERIC_EXPORT_H
 

include/linux/export.h

+/* SPDX-License-Identifier: GPL-2.0-only */
 #ifndef _LINUX_EXPORT_H
 #define _LINUX_EXPORT_H
 

include/linux/license.h

+/* SPDX-License-Identifier: GPL-2.0-only */
 #ifndef __LICENSE_H
 #define __LICENSE_H
 

include/linux/module.h

-#ifndef _LINUX_MODULE_H
-#define _LINUX_MODULE_H
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
  * Dynamic loading of modules into the kernel.
  *
  * Rewritten by Richard Henderson <rth@tamu.edu> Dec 1996
  * Rewritten again by Rusty Russell, 2002
  */
+
+#ifndef _LINUX_MODULE_H
+#define _LINUX_MODULE_H
+
 #include <linux/list.h>
 #include <linux/stat.h>
 #include <linux/compiler.h>