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.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>