selftests: vDSO: vdso_test_getrandom: Avoid -Wunused

vgetrandom_put_state() and the variable "ret" in kselftest() are unused.

Drop the variable "ret". Suppress the warning for  vgetrandom_put_state()
as it is meant as an example for libc implementors.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250611-selftests-vdso-fixes-v3-4-e62e37a6bcf5@linutronix.de