selftests: vDSO: vdso_test_correctness: Fix -Wstrict-prototypes

Functions definitions without any argument list produce a warning with
-Wstrict-prototypes:

vdso_test_correctness.c:111:13: warning: function declaration isn’t a prototype [-Wstrict-prototypes]
  111 | static void fill_function_pointers()
      |             ^~~~~~~~~~~~~~~~~~~~~~

Explicitly use an empty argument list.

Now that all selftests a free of this warning, enable it in the Makefile.

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-v3-7-e62e37a6bcf5@linutronix.de