selftests: vDSO: vdso_config: Avoid -Wunused-variables

Not all users of this header make use of all its variables.
For example vdso_test_correctness.c does not use "versions":

In file included from vdso_test_correctness.c:22:
vdso_config.h:61:20: warning: ‘versions’ defined but not used [-Wunused-variable]
   61 | static const char *versions[7] = {
      |                    ^~~~~~~~

Avoid those warnings through attribute((unused)).

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/all/20250611-selftests-vdso-fixes-v3-5-e62e37a6bcf5@linutronix.de