x86, xen, vdso: fix build error

fix:

   arch/x86/xen/built-in.o: In function `xen_enable_syscall':
   (.cpuinit.text+0xdb): undefined reference to `sysctl_vsyscall32'
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/setup.c

@@ -144,7 +144,9 @@ void __cpuinit xen_enable_syscall(void)
 		if (ret != 0) {
 			printk(KERN_INFO "Xen: 32-bit syscall not supported: disabling vdso\n");
 			setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
+#ifdef CONFIG_COMPAT
 			sysctl_vsyscall32 = 0;
+#endif
 		}
 	}
 #endif /* CONFIG_X86_64 */