x86/headers: Remove <asm/sigcontext.h> references on the kernel side

Now that all type definitions are in the UAPI header, include it
directly, instead of through <asm/sigcontext.h>.

[ We still keep asm/sigcontext.h, so that uapi/asm/sigcontext32.h
  can include <asm/sigcontext.h>. ]

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/1441438363-9999-16-git-send-email-mingo@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>