Replace luaL_reg with luaL_Reg as recent LuaJIT dropped the Lua 5.0 compat (#5541)

We are bundling Lua5.1 which has same macro

src/script/lua_api/l_object.h

@@ -37,7 +37,7 @@ class ObjectRef : public ModApiBase {
 	ServerActiveObject *m_object;
 
 	static const char className[];
-	static const luaL_reg methods[];
+	static const luaL_Reg methods[];
 public:
 	static ObjectRef *checkobject(lua_State *L, int narg);