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

Author: t-8ch

tools/testing/selftests/vDSO/Makefile

@@ -12,7 +12,7 @@ TEST_GEN_PROGS += vdso_test_correctness
 TEST_GEN_PROGS += vdso_test_getrandom
 TEST_GEN_PROGS += vdso_test_chacha
 
-CFLAGS := -std=gnu99 -O2 -Wall
+CFLAGS := -std=gnu99 -O2 -Wall -Wstrict-prototypes
 
 ifeq ($(CONFIG_X86_32),y)
 LDLIBS += -lgcc_s

tools/testing/selftests/vDSO/vdso_test_correctness.c

@@ -108,7 +108,7 @@ static void *vsyscall_getcpu(void)
 }
 
 
-static void fill_function_pointers()
+static void fill_function_pointers(void)
 {
 	void *vdso = dlopen("linux-vdso.so.1",
 			    RTLD_LAZY | RTLD_LOCAL | RTLD_NOLOAD);