bdk: lvgl: correct slider knob signal coordinates

It should always point to the middle of the knob and not be variable with offset.

bdk/libs/lvgl/lv_objx/lv_line.c

@@ -1,3 +1,19 @@
+/*
+ * Copyright (c) 2019 CTCaer
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
 /**
  * @file lv_line.c
  *