Add licence notice to source

Signed-off-by: Mahyar Koshkouei <mahyar.koshkouei@gmail.com>

mpv-libretro.c

@@ -1,3 +1,20 @@
+/* mpv media player libretro core
+ * Copyright (C) 2018 Mahyar Koshkouei
+ * 
+ * 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/>.
+ */
+
 #include <stdarg.h>
 #include <stdio.h>
 #include <string.h>