vdso: Add struct __kernel_old_timeval forward declaration to gettime.h

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: 42874e4e ("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