sdbus++: async: server: add generator stub

Add the needful changes in the tools and meson-generator to
create an asynchronous server binding header.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I0607aefc006eafb9224d92b887cf2d3b30c8da06
diff --git a/test/gen/server/Test/meson.build b/test/gen/server/Test/meson.build
index 829f238..459f0a8 100644
--- a/test/gen/server/Test/meson.build
+++ b/test/gen/server/Test/meson.build
@@ -2,7 +2,7 @@
 generated_sources += custom_target(
     'server/Test__cpp'.underscorify(),
     input: [ '../../../yaml/server/Test.interface.yaml',  ],
-    output: [ 'common.hpp', 'server.cpp', 'server.hpp', 'client.hpp',  ],
+    output: [ 'common.hpp', 'server.cpp', 'server.hpp', 'aserver.hpp', 'client.hpp',  ],
     depend_files: sdbusplusplus_depfiles,
     command: [
         sdbuspp_gen_meson_prog, '--command', 'cpp',