materia-theme: fix build

I'm not sure why this was disabled, but when we later ran
"ninja install", it would be built anyway.  Now that we run
"meson install", that's no longer implicit, so we have to have the
build step enabled.

Fixes: 10f35ff05d ("meson.setupHook: prefer meson commands over ninja")

pkgs/data/themes/materia-theme/default.nix

@@ -27,8 +27,6 @@ stdenv.mkDerivation rec {
 
   propagatedUserEnvPkgs = [ gtk-engine-murrine ];
 
-  dontBuild = true;
-
   mesonFlags = [
     "-Dgnome_shell_version=${lib.versions.majorMinor gnome.gnome-shell.version}"
   ];