gcompris: add extra-cmake-modules

This was removed in #401292, and since then the shortcut disappeared
from the launcher, because `share/applications/org.kde.gcompris.desktop`
installation is conditioned to ECM:
https://invent.kde.org/education/gcompris/-/blob/master/CMakeLists.txt?ref_type=heads#L142

pkgs/games/gcompris/default.nix

···
   qtmultimedia,
   qtsensors,
   qttools,
+  extra-cmake-modules,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
···
 
   nativeBuildInputs = [
     cmake
+    extra-cmake-modules
     gettext
     ninja
     qttools