AlsaTools: Add license

pkgs/os-specific/linux/alsa-tools/default.nix

···
     done
   '';
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = http://www.alsa-project.org/;
     description = "ALSA, the Advanced Linux Sound Architecture tools";
 
···
       MIDI functionality to the Linux-based operating system.
     '';
 
-    platforms = stdenv.lib.platforms.linux;
-    maintainers = [ stdenv.lib.maintainers.fps ];
+    license = licenses.gpl2;
+    platforms = platforms.linux;
+    maintainers = [ maintainers.fps ];
   };
 }