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 <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Link: http://lkml.kernel.org/r/CA%2B55aFxsOMc9=p02r8-QhJ=h=[e-mail]
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/mm/fault.c

···
 #include <asm/traps.h>			/* dotraplinkage, ...		*/
 #include <asm/pgalloc.h>		/* pgd_*(), ...			*/
 #include <asm/kmemcheck.h>		/* kmemcheck_*(), ...		*/
+#include <asm/fixmap.h>			/* VSYSCALL_START		*/
 
 /*
  * Page fault error code bits: