Fix differing tag (warning on MSVC)

src/dds/build/plan/compile_file.hpp

     using runtime_error::runtime_error;
 };
 
-struct shared_compile_file_rules {
+class shared_compile_file_rules {
     struct rules_impl {
         std::vector<fs::path>    inc_dirs;
         std::vector<std::string> defs;