alpha: Fix build around srm_sysrq_reboot_op

The patch introducing the struct was probably never compile tested,
because it sets a handler with a wrong function signature. Wrap the
handler into a functions with the correct signature to fix the build.

Fixes: 0f1c9688 ("tty/sysrq: alpha: export and use __sysrq_get_key_op()")
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: NGuenter Roeck <linux@roeck-us.net>
Tested-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NJoerg Roedel <jroedel@suse.de>
Signed-off-by: NMatt Turner <mattst88@gmail.com>

arch/alpha/kernel/setup.c

@@ -430,8 +430,13 @@ register_cpus(void)
 arch_initcall(register_cpus);
 
 #ifdef CONFIG_MAGIC_SYSRQ
+static void sysrq_reboot_handler(int unused)
+{
+	machine_halt();
+}
+
 static const struct sysrq_key_op srm_sysrq_reboot_op = {
-	.handler	= machine_halt,
+	.handler	= sysrq_reboot_handler,
 	.help_msg       = "reboot(b)",
 	.action_msg     = "Resetting",
 	.enable_mask    = SYSRQ_ENABLE_BOOT,