x86-32: Fix build failure with AUDIT=y, AUDITSYSCALL=n

JONGMAN HEO reports:

  With current linus git (commit a25a2b84), I got following build error,

  arch/x86/kernel/vm86_32.c: In function 'do_sys_vm86':
  arch/x86/kernel/vm86_32.c:340: error: implicit declaration of function '__audit_syscall_exit'
  make[3]: *** [arch/x86/kernel/vm86_32.o] Error 1

OK, I can reproduce it (32bit allmodconfig with AUDIT=y, AUDITSYSCALL=n)

It's due to commit d7e7528b: "Audit: push audit success and retcode
into arch ptrace.h".
Reported-by: JONGMAN HEO <jongman.heo@samsung.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/vm86_32.c

@@ -336,8 +336,10 @@ static void do_sys_vm86(struct kernel_vm86_struct *info, struct task_struct *tsk
 		mark_screen_rdonly(tsk->mm);
 
 	/*call __audit_syscall_exit since we do not exit via the normal paths */
+#ifdef CONFIG_AUDITSYSCALL
 	if (unlikely(current->audit_context))
 		__audit_syscall_exit(1, 0);
+#endif
 
 	__asm__ __volatile__(
 		"movl %0,%%esp\n\t"