s390/kexec_file: add declaration of purgatory related globals

Fix the following sparse complaints:

arch/s390/purgatory/purgatory.c:18:5: warning: symbol 'kernel_entry' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:19:5: warning: symbol 'kernel_type' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:21:5: warning: symbol 'crash_start' was not declared. Should it be static?
arch/s390/purgatory/purgatory.c:22:5: warning: symbol 'crash_size' was not declared. Should it be static?
Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/include/asm/purgatory.h

@@ -13,5 +13,11 @@
 
 int verify_sha256_digest(void);
 
+extern u64 kernel_entry;
+extern u64 kernel_type;
+
+extern u64 crash_start;
+extern u64 crash_size;
+
 #endif	/* __ASSEMBLY__ */
 #endif /* _S390_PURGATORY_H_ */