[S390] Add missing ifndef/define to include/asm-s390/sysinfo.h.

In order to protect against compile breakage in case the header file
gets included twice.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>