removed extern code generation declarations preferring direct includes (#7948)

* removed extern code generation definitions, preferring direct includes

* add static to functions

* remove idl_gen_lua

include/flatbuffers/code_generators.h

@@ -229,6 +229,10 @@ class TypedFloatConstantGenerator : public FloatConstantGenerator {
   const std::string neg_inf_number_;
 };
 
+std::string JavaCSharpMakeRule(const bool java, const Parser &parser,
+                               const std::string &path,
+                               const std::string &file_name);
+
 }  // namespace flatbuffers
 
 #endif  // FLATBUFFERS_CODE_GENERATORS_H_