Add Apache license header back to libpqueue files

The original libpqueue file were licensed under Apache 2.0 so
therefore should retain their copyrights and header as per the
license terms at http://www.apache.org/licenses/LICENSE-2.0

src/pqueue.c

@@ -3,6 +3,28 @@
  *
  * This file is part of libgit2, distributed under the GNU GPL v2 with
  * a Linking Exception. For full terms see the included COPYING file.
+ *
+ * This file is based on a modified version of the priority queue found
+ * in the Apache project and libpqueue library.
+ * 
+ * https://github.com/vy/libpqueue
+ * 
+ * Original file notice:
+ * 
+ * Copyright 2010 Volkan Yazici <volkan.yazici@gmail.com>
+ * Copyright 2006-2010 The Apache Software Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
  */
 
 #include "common.h"

src/pqueue.h

 /*
- * Copyright (C) 2009-2012 the libgit2 contributors
+ * Copyright (C) 2009-2013 the libgit2 contributors
  *
  * This file is part of libgit2, distributed under the GNU GPL v2 with
  * a Linking Exception. For full terms see the included COPYING file.
+ * 
+ * This file is based on a modified version of the priority queue found
+ * in the Apache project and libpqueue library.
+ * 
+ * https://github.com/vy/libpqueue
+ * 
+ * Original file notice:
+ * 
+ * Copyright 2010 Volkan Yazici <volkan.yazici@gmail.com>
+ * Copyright 2006-2010 The Apache Software Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
  */
 
 #ifndef INCLUDE_pqueue_h__