s390/topology: remove sysinfo header include, add forward declaration instead

Any change to sysinfo.h causes a whole kernel recompile since sysinfo.h is
included by topology.h, which again is used nearly everywhere.
So remove that include and add a forward declaration instead.

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