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 <[e-mail]>
Fixes: 1a280f48c0e4 ("s390/kprobes: replace kretprobe with rethook")
Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Heiko Carstens <[e-mail]>

arch/s390/include/asm/kprobes.h

···
 };
 
 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