The prototype of __vdso_gettimeofday() uses this struct. However
gettime.h's own includes do not provide a definition for it.
Add a forward declaration, similar to other used structs.
Fixes: 42874e4eb3 ("arch: vdso: consolidate gettime prototypes")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250815-vdso-sparc64-generic-2-v2-1-b5ff80672347@linutronix.de
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