x86: fix mm/fault.c build

arch/x86/mm/fault.c needs to include asm/vsyscall.h to fix a
build error:

  arch/x86/mm/fault.c: In function '__bad_area_nosemaphore':
  arch/x86/mm/fault.c:728: error: 'VSYSCALL_START' undeclared (first use in this function)

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/mm/fault.c

@@ -17,6 +17,7 @@
 #include <asm/traps.h>			/* dotraplinkage, ...		*/
 #include <asm/pgalloc.h>		/* pgd_*(), ...			*/
 #include <asm/kmemcheck.h>		/* kmemcheck_*(), ...		*/
+#include <asm/vsyscall.h>
 
 /*
  * Page fault error code bits: