serial: clps711x: Fix COMPILE_TEST build for target without GPIOLIB support

The patch fixes the following build error of CLPS711X serial driver for
targets without GPIOLIB support:

>> drivers/tty/serial/serial_mctrl_gpio.c:44:6: error: redefinition of 'mctrl_gpio_set'
  void mctrl_gpio_set(struct mctrl_gpios *gpios, unsigned int mctrl)
      ^
  In file included from drivers/tty/serial/serial_mctrl_gpio.c:23:0:
  drivers/tty/serial/serial_mctrl_gpio.h:80:6: note: previous definition of 'mctrl_gpio_set' was here
  void mctrl_gpio_set(struct mctrl_gpios *gpios, unsigned int mctrl)
      ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -222,7 +222,7 @@ config SERIAL_CLPS711X
 	tristate "CLPS711X serial port support"
 	depends on ARCH_CLPS711X || COMPILE_TEST
 	select SERIAL_CORE
-	select SERIAL_MCTRL_GPIO
+	select SERIAL_MCTRL_GPIO if GPIOLIB
 	help
 	  This enables the driver for the on-chip UARTs of the Cirrus
 	  Logic EP711x/EP721x/EP731x processors.