time_namespace: always provide arch_get_vdso_data() prototype for vdso

The arch_get_vdso_data() function is defined separately on each
architecture, but only called when CONFIG_TIME_NS is set. If the
definition is a global function, this causes a W=1 warning without
TIME_NS:

arch/x86/entry/vdso/vma.c:35:19: error: no previous prototype for 'arch_get_vdso_data' [-Werror=missing-prototypes]

Move the prototype out of the #ifdef block to reliably turn off that
warning.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnd Bergmann <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Eric Paris <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Michal Simek <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: Pavel Machek <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/time_namespace.h

···
 				    struct time_namespace *old_ns);
 void free_time_ns(struct time_namespace *ns);
 void timens_on_fork(struct nsproxy *nsproxy, struct task_struct *tsk);
-struct vdso_data *arch_get_vdso_data(void *vvar_page);
 struct page *find_timens_vvar_page(struct vm_area_struct *vma);
 
 static inline void put_time_ns(struct time_namespace *ns)
···
 	return tim;
 }
 #endif
+
+struct vdso_data *arch_get_vdso_data(void *vvar_page);
 
 #endif /* _LINUX_TIMENS_H */