s390/rethook: add local rethook header file

Compiling the kernel with CONFIG_KPROBES disabled, but CONFIG_RETHOOK
enabled, results in this sparse warning:

arch/s390/kernel/rethook.c:26:15: warning: no previous prototype for 'arch_rethook_trampoline_callback' [-Wmissing-prototypes]
    26 | unsigned long arch_rethook_trampoline_callback(struct pt_regs *regs)
       |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Add a local rethook header file similar to riscv to address this.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 1a280f48 ("s390/kprobes: replace kretprobe with rethook")
Link: https://lore.kernel.org/all/202302030102.69dZIuJk-lkp@intel.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/kprobes.h

@@ -70,7 +70,6 @@ struct kprobe_ctlblk {
 };
 
 void arch_remove_kprobe(struct kprobe *p);
-unsigned long arch_rethook_trampoline_callback(struct pt_regs *regs);
 
 int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 int kprobe_exceptions_notify(struct notifier_block *self,

arch/s390/kernel/rethook.c

 // SPDX-License-Identifier: GPL-2.0-or-later
 #include <linux/rethook.h>
 #include <linux/kprobes.h>
+#include "rethook.h"
 
 void arch_rethook_prepare(struct rethook_node *rh, struct pt_regs *regs, bool mcount)
 {

arch/s390/kernel/rethook.h

+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __S390_RETHOOK_H
+#define __S390_RETHOOK_H
+
+unsigned long arch_rethook_trampoline_callback(struct pt_regs *regs);
+
+#endif