alsa-plugins: provide more precise licence information

The code is licenced under LGPL 2.1 or later with the follawing
exceptions:

- The maemo plugin is licenced under the older LGPL 2 (or later), and
- m4/attributes.m4 and usb_stream/usb_stream.h are marked as being
GPL 2 or later.

I assume that in most jurisdiction the inclusion of a single
GPL‐licenced header would not be sufficient to make the whole package
or even just the usb_stream plugin a derivative work, but I am not a
lawyer and think that it is best to expose precise licence information.

pkgs/by-name/al/alsa-plugins/package.nix

···
   meta = {
     description = "Various plugins for ALSA";
     homepage = "http://alsa-project.org/";
-    license = lib.licenses.lgpl21;
+
+    license = with lib.licenses; [
+      lgpl21Plus
+      lgpl2Plus # maemo plugin
+      gpl2Plus # attributes.m4 & usb_stream.h
+    ];
+
     maintainers = [ lib.maintainers.marcweber ];
     platforms = lib.platforms.linux;
   };