Clarify license (GPLv3+)

bbswitch.c

@@ -9,6 +9,20 @@
  * Get status
  * # cat /proc/acpi/bbswitch
  */
+/*
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that 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/>.
+ */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/pci.h>