USB: serial: usb_wwan: Add missing uaccess.h / fix build failure

This patch fixes a build failure[1] by adding the missing uaccess.h needed
for copy_from_user and copy_to_user

References:
	http://kisskb.ellerman.id.au/kisskb/buildresult/3607218/Signed-off-by: NPeter Huewe <peterhuewe@gmx.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb_wwan.c

@@ -31,6 +31,7 @@
 #include <linux/tty_flip.h>
 #include <linux/module.h>
 #include <linux/bitops.h>
+#include <linux/uaccess.h>
 #include <linux/usb.h>
 #include <linux/usb/serial.h>
 #include <linux/serial.h>