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/example/gen/meson.build b/example/gen/meson.build
index 23394ac..b550c79 100644
--- a/example/gen/meson.build
+++ b/example/gen/meson.build
@@ -5,10 +5,10 @@
     check: true,
 ).stdout().strip().split('\n')[0]
 
-if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 6'
+if sdbuspp_gen_meson_ver != 'sdbus++-gen-meson version 7'
     warning('Generated meson files from wrong version of sdbus++-gen-meson.')
     warning(
-        'Expected "sdbus++-gen-meson version 6", got:',
+        'Expected "sdbus++-gen-meson version 7", got:',
         sdbuspp_gen_meson_ver
     )
 endif