USB: serial: metro-usb: add to the build

This adds the metro-usb driver to the build system properly.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/usb/serial/Makefile b/drivers/usb/serial/Makefile
index 5ac438b..07f198e 100644
--- a/drivers/usb/serial/Makefile
+++ b/drivers/usb/serial/Makefile
@@ -37,6 +37,7 @@
 obj-$(CONFIG_USB_SERIAL_KLSI)			+= kl5kusb105.o
 obj-$(CONFIG_USB_SERIAL_KOBIL_SCT)		+= kobil_sct.o
 obj-$(CONFIG_USB_SERIAL_MCT_U232)		+= mct_u232.o
+obj-$(CONFIG_USB_SERIAL_METRO)			+= metro-usb.o
 obj-$(CONFIG_USB_SERIAL_MOS7720)		+= mos7720.o
 obj-$(CONFIG_USB_SERIAL_MOS7840)		+= mos7840.o
 obj-$(CONFIG_USB_SERIAL_MOTOROLA)		+= moto_modem.o