a52: explicitly mark module as GPL

Following discussions from !6043, I've been looking at determining the license of a given DLL post-build, notably using strings liba52_plugin.dll | grep "GNU General Public License"

This works fine for explicitly marked GPL modules, but not for LGPL modules with GPL contribs making them de facto GPL modules. The current state is kind of misleading and I'd like to correct it for all modules involved, which contain the following text:

NOTA BENE: this module requires the linking against a library which is known to require licensing under the GNU General Public License version 2 (or later). Therefore, the result of compiling this module will normally be subject to the terms of that later license.

If you're OK with this, I'll update the MR with the remaining modules that will be affected by this change.