gdb: fix building without UART support

GDB can now also be built without UART support, e.g. on Xtensa it can
use a memory window backend. Update the Kconfig dependency to avoid
build failures.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

subsys/debug/gdbstub/Kconfig

@@ -4,7 +4,7 @@
 menuconfig GDBSTUB
 	bool "GDB remote serial protocol support [EXPERIMENTAL]"
 	depends on ARCH_HAS_GDBSTUB
-	select UART_USE_RUNTIME_CONFIGURE
+	select UART_USE_RUNTIME_CONFIGURE if SERIAL
 	select EXPERIMENTAL
 	help
 	  This option enable support the target using GDB, or any other