guitone: add license

pkgs/applications/version-management/guitone/default.nix

···
 
   meta = {
     description = "Qt4 based GUI for monotone";
-    homepage = http://guitone.thomaskeller.biz;
+    homepage = https://guitone.thomaskeller.biz;
     downloadPage = https://code.monotone.ca/p/guitone/;
+    license = stdenv.lib.licenses.gpl3;
     inherit (qt4.meta) platforms;
   };
 }