x86/entry: Fix allnoconfig build warning

The following commit:

  095b7a3e7745 ("x86/entry: Convert double fault exception to IDTENTRY_DF")

introduced a new build warning on 64-bit allnoconfig kernels, that have CONFIG_VMAP_STACK disabled:

  arch/x86/kernel/traps.c:332:16: warning: unused variable ‘address’ [-Wunused-variable]

This variable is only used if CONFIG_VMAP_STACK is defined, so make it
dependent on that, not CONFIG_X86_64.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>

arch/x86/kernel/traps.c

@@ -328,7 +328,7 @@ DEFINE_IDTENTRY_DF(exc_double_fault)
 	static const char str[] = "double fault";
 	struct task_struct *tsk = current;
 
-#ifdef CONFIG_X86_64
+#ifdef CONFIG_VMAP_STACK
 	unsigned long address = read_cr2();
 #endif