selftests/pidfd: Fix config for pidfd_setns_test

Required by switch_timens() to open /proc/self/ns/time_for_children.

CONFIG_GENERIC_VDSO_TIME_NS is not available on UML, so pidfd_setns_test
cannot be run successfully on this architecture.

Cc: Shuah Khan <skhan@linuxfoundation.org>
Fixes: 2b40c5db

 ("selftests/pidfd: add pidfd setns tests")
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20240511171445.904356-2-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>