vdso: Gate VDSO_GETRANDOM behind HAVE_GENERIC_VDSO

All architectures which want to implement getrandom() in the vDSO need to
use the generic vDSO library.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250826-vdso-cleanups-v1-11-d9b65750e49f@linutronix.de

Author: t-8ch

lib/vdso/Kconfig

@@ -19,9 +19,9 @@ config GENERIC_VDSO_OVERFLOW_PROTECT
 	  time getter functions for the price of an extra conditional
 	  in the hotpath.
 
-endif
-
 config VDSO_GETRANDOM
 	bool
 	help
 	  Selected by architectures that support vDSO getrandom().
+
+endif