drdb: add license

pkgs/os-specific/linux/drbd/default.nix

···
 
   installFlags = "localstatedir=$(TMPDIR)/var sysconfdir=$(out)/etc INITDIR=$(out)/etc/init.d";
 
-  meta = {
+  meta = with stdenv.lib; {
     homepage = http://www.drbd.org/;
     description = "Distributed Replicated Block Device, a distributed storage system for Linux";
-    platforms = stdenv.lib.platforms.linux;
+    license = licenses.gpl2;
+    platforms = platforms.linux;
   };
 }