extable.h: add stddef.h so "NULL" definition is not implicit

While not an issue now, eventually we will have independent users of the extable.h file and we will stop sourcing it via module.h header.

In testing that pending work, with very sparse builds, characteristic of an "allnoconfig" on various architectures, we can sometimes hit an instance where the very basic standard definitions aren't present, resulting in:

include/linux/extable.h:26:9: error: 'NULL' undeclared (first use in this function)

To be clear, this isn't a regression, since currently extable.h is only used by module.h -- however, we will need this addition present before we start migrating exception table users off module.h and onto extable.h during the next release cycle.

---

[Was not expecting extable.h to be in 4.8 content, but since it is, it
might as well have this one line fixup to make it ready for 4.9 ; feel free to squash this into the original extable.h creation if rebasing.]

 include/linux/extable.h | 2 ++
 1 file changed, 2 insertions(+)

index 2c71dccd1bc3..7effea4b257d 100644
@@ -1,6 +1,8 @@
 #ifndef _LINUX_EXTABLE_H
 #define _LINUX_EXTABLE_H
 
+#include <linux/stddef.h>	/* for NULL */
+
 struct module;
 struct exception_table_entry;
 

2.8.4

Applied. I prefer not to rebase published trees, and it's not like it's going to break anyone's bisect.

Thanks,
Rusty.