s390/kexec: add missing include to machine_kexec_reloc.c

Include <asm/kexec.h> into machine_kexec_reloc.c to expose
arch_kexec_do_relocs declaration and avoid the following sparse warnings:
arch/s390/kernel/machine_kexec_reloc.c:4:5: warning: symbol 'arch_kexec_do_relocs' was not declared. Should it be static?
arch/s390/boot/../kernel/machine_kexec_reloc.c:4:5: warning: symbol 'arch_kexec_do_relocs' was not declared. Should it be static?
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/machine_kexec_reloc.c

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/elf.h>
+#include <asm/kexec.h>
 
 int arch_kexec_do_relocs(int r_type, void *loc, unsigned long val,
 			 unsigned long addr)