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
void main() {}

Reported-by: Josh Boyer
Signed-off-by: Denys Vlasenko
Cc: Andrew Morton
Cc: Arnd Bergmann
Cc: David Rientjes
Cc: Linus Torvalds
Cc: Peter Zijlstra
Cc: Thomas Gleixner
Cc: Thomas Graf
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1459289697-12875-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar