s390/vdso: add missing boot_vdso_data declaration

sparse says:
arch/s390/kernel/vdso.c:150:18:
 warning: symbol 'boot_vdso_data' was not declared. Should it be static?
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/include/asm/vdso.h

@@ -46,6 +46,7 @@ struct vdso_per_cpu_data {
 };
 
 extern struct vdso_data *vdso_data;
+extern struct vdso_data boot_vdso_data;
 
 void vdso_alloc_boot_cpu(struct lowcore *lowcore);
 int vdso_alloc_per_cpu(struct lowcore *lowcore);