fvwm: updating license to gplv2+

I looked at some src, and it mentions "or later".

pkgs/applications/window-managers/fvwm/default.nix

···
   meta = {
     homepage = "http://fvwm.org";
     description = "A multiple large virtual desktop window manager";
-    license = "GPLv2";
+    license = "GPLv2+";
   };
 }