Lock uwsm desktop file behind feature flag

The file in the repo cannot be used in NixOS due to missing full paths,
and the fact that `uwsm` does not have access to `PATH` to find the
listed binaries. Might be useful in other situations as well.

systemd/meson.build

@@ -1,5 +1,7 @@
-install_data(                                                          
-  'hyprland-uwsm.desktop',                                                  
-  install_dir: join_paths(get_option('datadir'), 'wayland-sessions'),  
-  install_tag: 'runtime',                                              
-)                                                                      
+if (get_option('uwsm').allowed())
+  install_data(
+    'hyprland-uwsm.desktop',
+    install_dir: join_paths(get_option('datadir'), 'wayland-sessions'),
+    install_tag: 'runtime',
+  )
+endif