alsa-utils: provide precise licence information

Most of the code is licenced under GPL 2 or later, with the following
exceptions:

- alsactl is mostly GPL 2+ with init_parse.c, init_sysdeps.c,
  init_sysdeps.c, init_utils_run.c and init_utils_string.c being marked
  as GPL 2 only, and
- alsaucm is LGPL 2.1 or later.

I think it is safe to assume that following explicit licence
compatibility clauses the complete package can be distributed under
the terms of GPL version 2, but this commit exposes the licence
information as precisely as possible.

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

@@ -87,7 +87,12 @@ stdenv.mkDerivation (finalAttrs: {
       MIDI functionality to the Linux-based operating system.
     '';
 
-    license = lib.licenses.gpl2;
+    license = with lib.licenses; [
+      gpl2Plus
+      gpl2Only # alsactl (init_{parse,sysdeps,sysfs,utils_{run,string}}.c, rest GPL 2.0+)
+      lgpl21Plus # alsaucm
+    ];
+
     platforms = lib.platforms.linux;
     maintainers = [ ];
   };