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>