serial: mctrl_gpio: Add missing module license

As the mctrl_gpio driver can be built as a module, it needs to have its
license specified with MODULE_LICENSE. Otherwise, it cannot access
required symbols exported through EXPORT_SYMBOL_GPL.
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_mctrl_gpio.c

@@ -20,6 +20,7 @@
 #include <linux/gpio/consumer.h>
 #include <linux/termios.h>
 #include <linux/serial_core.h>
+#include <linux/module.h>
 
 #include "serial_mctrl_gpio.h"
 
@@ -249,3 +250,5 @@ void mctrl_gpio_disable_ms(struct mctrl_gpios *gpios)
 	}
 }
 EXPORT_SYMBOL_GPL(mctrl_gpio_disable_ms);
+
+MODULE_LICENSE("GPL");