uapi/linux/stddef.h: Provide __always_inline to userspace headers

uapi/linux/stddef.h: Provide __always_inline to userspace headers

Josh Boyer reported that my recent change to uapi/linux/swab.h broke the Qemu build:

  bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of some byteswap operations")

Unfortunately, UAPI headers don't include compiler.h so fixing it there is not enough,
add an __always_inline definition to uapi/linux/stddef.h instead.

Testcase: "make headers_install" and try to compile this:

	#include <linux/swab.h>
	void main() {}

Reported-by: Josh Boyer <[email protected]>
Signed-off-by: Denys Vlasenko <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Thomas Graf <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>