Header-comments cleanup. Unused includes cleanup

[inobelar]

The reason for this 'cleanup' PR, is minor garbage in header-comments, due (as I can see) to project initially generated from template.

Changes:

1. AUTHORS file moved from gst-h264-encryption directory to root
2. COPYING in gst-h264-encryption directory removed, since exactly the same placed in root directory (so now its not duplicated)
3. As header-comment used everywhere (except files of AES lib):

   /*
    * GStreamer H264 Encryption Plugin
    *
    * Copyright (C) 2024 Oguzhan Oztaskin <[email protected]>
    *
    * 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 <https://www.gnu.org/licenses/>.
    */

   • Since, as I can see, you choosed GPLv3. Template for it given from here
   • Inspired by GStreamer's small GPLv2 header comments, like in gstudp
4. Removed unused includes in some places
5. Some includes was annotated with #include <xxx.h> // for: yyy for clarity
6. Improved readability of some 'include guards', for example: __GST_H264ENCRYPTIONBASEPRIVATE_H__ -> __GST_H264_ENCRYPTION_BASE_PRIVATE_H__
7. Removed 'extra' semicolons in G_DEFINE_TYPE(...), GST_ELEMENT_REGISTER_DEFINE() macro
8. Changed project name in meson.build from gst-template to gst-h264-encryption