selftests: vDSO: Enable -Wall

Protect against common programming errors through compiler warnings.
These warnings are also used for the kernel itself.

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-v3-6-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
+CFLAGS := -std=gnu99 -O2 -Wall
 
 ifeq ($(CONFIG_X86_32),y)
 LDLIBS += -lgcc_s