HID: wacom: wacom_setup_numbered_buttons is local to wacom_wac

wacom_setup_numbered_buttons() is not used outside of wacom_wac.c,
make it static.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index 391a687..c40a6d1 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -2711,7 +2711,7 @@
 	return 0;
 }
 
-void wacom_setup_numbered_buttons(struct input_dev *input_dev,
+static void wacom_setup_numbered_buttons(struct input_dev *input_dev,
 				int button_count)
 {
 	int i;