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:

  bc27fb68aa ("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 <jwboyer@fedoraproject.org>
Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Thomas Graf <tgraf@suug.ch>
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 <mingo@kernel.org>

include/uapi/linux/stddef.h

@@ -1 +1,5 @@
 #include <linux/compiler.h>
+
+#ifndef __always_inline
+#define __always_inline inline
+#endif