extra-cmake-modules: add top-level package

This package no longer depends on Qt 5, but it is part of the KDE
Frameworks collection that do (so it is exported from there).

pkgs/top-level/all-packages.nix

@@ -6264,6 +6264,9 @@ in
 
   cmakeWithGui = self.cmakeCurses.override { useQt4 = true; };
 
+  # Does not actually depend on Qt 5
+  extra-cmake-modules = qt5.extra-cmake-modules;
+
   coccinelle = callPackage ../development/tools/misc/coccinelle { };
 
   cpptest = callPackage ../development/libraries/cpptest { };