time: Build generic update_vsyscall() only with generic time vDSO

The generic vDSO can be used without the time-related functionality.
In that case the generic update_vsyscall() from kernel/time/vsyscall.c
should not be built.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250826-vdso-cleanups-v1-5-d9b65750e49f@linutronix.de

Author: t-8ch

kernel/time/Makefile

@@ -26,7 +26,7 @@ obj-$(CONFIG_LEGACY_TIMER_TICK)			+= tick-legacy.o
 ifeq ($(CONFIG_SMP),y)
  obj-$(CONFIG_NO_HZ_COMMON)			+= timer_migration.o
 endif
-obj-$(CONFIG_HAVE_GENERIC_VDSO)			+= vsyscall.o
+obj-$(CONFIG_GENERIC_GETTIMEOFDAY)		+= vsyscall.o
 obj-$(CONFIG_DEBUG_FS)				+= timekeeping_debug.o
 obj-$(CONFIG_TEST_UDELAY)			+= test_udelay.o
 obj-$(CONFIG_TIME_NS)				+= namespace.o