Add another license header

* Missed this one in the first sweep

Change-Id: I5f2753f93ce3e9888c553de4c2e510137e818a29

gui/checkbox.cpp

@@ -1,3 +1,21 @@
+/*
+	Copyright 2017 TeamWin
+	This file is part of TWRP/TeamWin Recovery Project.
+
+	TWRP 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.
+
+	TWRP 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 TWRP.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
 // checkbox.cpp - GUICheckbox object
 
 #include <stdarg.h>