kexec: Fix file verification on S390

I accidentally typoed this #ifdef, so verification would always be
disabled.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Reported-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

arch/s390/kernel/kexec_elf.c

@@ -130,7 +130,7 @@ static int s390_elf_probe(const char *buf, unsigned long len)
 const struct kexec_file_ops s390_kexec_elf_ops = {
 	.probe = s390_elf_probe,
 	.load = s390_elf_load,
-#ifdef CONFIG_KEXEC__SIG
+#ifdef CONFIG_KEXEC_SIG
 	.verify_sig = s390_verify_sig,
 #endif /* CONFIG_KEXEC_SIG */
 };