x86/headers: Fix (old) header file dependency bug in uapi/asm/sigcontext32.h

x86/headers: Fix (old) header file dependency bug in uapi/asm/sigcontext32.h

Mikko Rapeli reported that the following standalone user-space
header does not compile:

  #include <asm/sigcontext32.h>

Due to undefined 'struct __fpx_sw_bytes' which is defined in
asm/sigcontext.h.

The following header order works:

  #include <asm/sigcontext.h>
  #include <asm/sigcontext32.h>

and that's probably how everyone's been using these headers for
the past decade or so, but it's a legit header file dependency
bug, so include asm/sigcontext.h in sigcontext32.h to allow it
to be built standlone.

Reported-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Acked-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/r/1441438363-9999-2-git-send-email-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>