add: modbus,添加modbus库的注册函数声明

luat/include/luat_libs.h

@@ -218,4 +218,7 @@ LUAMOD_API int luaopen_icmp( lua_State *L );
 // bluetooth
 LUAMOD_API int luaopen_bluetooth( lua_State *L );
 
+// modbus
+LUAMOD_API int luaopen_modbus( lua_State *L );
+
 #endif