arm64: arch_timer: avoid unused function warning

arm64_1188873_read_cntvct_el0() is protected by the correct
CONFIG_ARM64_ERRATUM_1188873 #ifdef, but the only reference to it is
also inside of an CONFIG_ARM_ARCH_TIMER_OOL_WORKAROUND section,
and causes a warning if that is disabled:

drivers/clocksource/arm_arch_timer.c:323:20: error: 'arm64_1188873_read_cntvct_el0' defined but not used [-Werror=unused-function]

Since the erratum requires that we always apply the workaround
in the timer driver, select that symbol as we do for SoC
specific errata.

Fixes: 95b861a4 ("arm64: arch_timer: Add workaround for ARM erratum 1188873")
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/Kconfig

@@ -484,6 +484,7 @@ config ARM64_ERRATUM_1024718
 config ARM64_ERRATUM_1188873
 	bool "Cortex-A76: MRC read following MRRC read of specific Generic Timer in AArch32 might give incorrect result"
 	default y
+	select ARM_ARCH_TIMER_OOL_WORKAROUND
 	help
 	  This option adds work arounds for ARM Cortex-A76 erratum 1188873