kexec: restore arch_kexec_kernel_image_probe declaration

arch_kexec_kernel_image_probe function declaration has been removed by
commit 9ec4ece ("kexec_file,x86,powerpc: factor out kexec_file_ops
functions").  Still this function is overridden by couple of architectures
and proper prototype declaration is therefore important, so bring it back.
This fixes the following sparse warning on s390:
arch/s390/kernel/machine_kexec_file.c:333:5: warning: symbol
'arch_kexec_kernel_image_probe' was not declared.  Should it be static?

Link: http://lkml.kernel.org/r/patch.git-ff1c9045ebdc.your-ad-here.call-01564402297-ext-5690@work.hours
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Dave Young <dyoung@redhat.com>
Reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kexec.h

@@ -183,6 +183,8 @@ int kexec_purgatory_get_set_symbol(struct kimage *image, const char *name,
 				   bool get_value);
 void *kexec_purgatory_get_symbol_addr(struct kimage *image, const char *name);
 
+int __weak arch_kexec_kernel_image_probe(struct kimage *image, void *buf,
+					 unsigned long buf_len);
 void * __weak arch_kexec_kernel_image_load(struct kimage *image);
 int __weak arch_kexec_apply_relocations_add(struct purgatory_info *pi,
 					    Elf_Shdr *section,