xen/keyhandler: add missing noreturn attribute

Function `reboot_machine' does not return, but lacks the `noreturn'
attribute.

Functions that never return should be declared with a `noreturn'
attribute.

The lack of `noreturn' causes a violation of MISRA C Rule 17.11 (not
currently accepted in Xen), and also Rule 2.1: "A project shall not
contain unreachable code". Depending on the compiler used and the
compiler optimization used, the lack of `noreturn' might lead to the
presence of unreachable code.

No functional change.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Signed-off-by: Victor Lira <victorm.lira@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

xen/common/keyhandler.c

@@ -251,7 +251,7 @@ static void cf_check dump_hwdom_registers(unsigned char key)
     }
 }
 
-static void cf_check reboot_machine(unsigned char key, bool unused)
+static void noreturn cf_check reboot_machine(unsigned char key, bool unused)
 {
     printk("'%c' pressed -> rebooting machine\n", key);
     machine_restart(0);