Add hello_cxx_module to demos target

The hello_cxx_module executable was being created when Seastar_MODULE is enabled
but wasn't added to the demos target, so it wouldn't be built when building demos.
Added the dependency so it's properly included in the demos build.

Co-authored-by: avikivity <[email protected]>

Author: Copilot

demos/CMakeLists.txt

@@ -131,4 +131,5 @@ if (Seastar_MODULE)
   target_link_libraries (hello_cxx_module
     PRIVATE
       seastar-module)
+  add_dependencies (demos hello_cxx_module)
 endif ()