s390/vdso: add missing prototypes for vdso functions

clang W=1 warns about missing prototypes:

>> arch/s390/kernel/vdso64/getcpu.c:8:5: warning: no previous prototype for function '__s390_vdso_getcpu' [-Wmissing-prototypes]
   int __s390_vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
       ^

Add a local header file in order to get rid of this warnings.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>