meson: fix include dir for protobuf generated headers

../src/vdc-connection.h:34:10: fatal error: 'messages/vdc-messages.pb.h' file not found

src/messages/meson.build

@@ -12,7 +12,7 @@ dssMessagesLibrary = library('messages', [
 )
 dssMessages = declare_dependency(
   link_with : dssMessagesLibrary,
-  include_directories : include_directories('.'),
+  include_directories : dssMessagesLibrary.private_dir_include(),
   dependencies : [
     protobuf,
   ]