s390/nospec: include cpu.h

Fix the following sparse warnings:
symbol 'cpu_show_spectre_v1' was not declared. Should it be static?
symbol 'cpu_show_spectre_v2' 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>