x86, vsyscall: Add missing <asm/fixmap.h> to arch/x86/mm/fault.c

arch/x86/mm/fault.c now depend on having the symbol VSYSCALL_START
defined, which is best handled by including <asm/fixmap.h> (it isn't
unreasonable we may want other fixed addresses in this file in the
future, and so it is cleaner than including <asm/vsyscall.h>
directly.)

This addresses an x86-64 allnoconfig build failure.  On other
configurations it was masked by an indirect path:

<asm/smp.h> -> <asm/apic.h> -> <asm/fixmap.h> -> <asm/vsyscall.h>

... however, the first such include is conditional on CONFIG_X86_LOCAL_APIC.

Originally-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/CA%2B55aFxsOMc9=p02r8-QhJ=h=Mqwckk4_Pnx9LQt5%2BfqMp_exQ@mail.gmail.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>