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>

arch/s390/kernel/vdso64/getcpu.c

@@ -4,6 +4,7 @@
 #include <linux/compiler.h>
 #include <linux/getcpu.h>
 #include <asm/timex.h>
+#include "vdso.h"
 
 int __s390_vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused)
 {

arch/s390/kernel/vdso64/vdso.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ARCH_S390_KERNEL_VDSO64_VDSO_H
+#define __ARCH_S390_KERNEL_VDSO64_VDSO_H
+
+#include <vdso/datapage.h>
+
+struct getcpu_cache;
+
+int __s390_vdso_getcpu(unsigned *cpu, unsigned *node, struct getcpu_cache *unused);
+int __s390_vdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz);
+int __s390_vdso_clock_gettime(clockid_t clock, struct __kernel_timespec *ts);
+int __s390_vdso_clock_getres(clockid_t clock, struct __kernel_timespec *ts);
+
+#endif /* __ARCH_S390_KERNEL_VDSO64_VDSO_H */

arch/s390/kernel/vdso64/vdso64_generic.c

 // SPDX-License-Identifier: GPL-2.0
 #include "../../../../lib/vdso/gettimeofday.c"
+#include "vdso.h"
 
 int __s390_vdso_gettimeofday(struct __kernel_old_timeval *tv,
 			     struct timezone *tz)