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 <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]

lib/vdso/Kconfig

···
 	  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