soc/intel/skylake: put back uart_debug.c into verstage

uart_debug.c was accidentally dropped in verstage in
64ce1d122c
(https://review.coreboot.org/17136). Fix that.

Change-Id: If37a028550d419bada80d157c4de02fd82d26c89
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17175
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)

src/soc/intel/skylake/Makefile.inc

@@ -30,6 +30,7 @@ bootblock-y += tsc_freq.c
 verstage-y += flash_controller.c
 verstage-y += monotonic_timer.c
 verstage-y += pch.c
+verstage-$(CONFIG_UART_DEBUG) += uart_debug.c
 verstage-y += pmutil.c
 verstage-y += bootblock/i2c.c