Fix compile error with GCC

src/main.c

@@ -548,6 +548,7 @@ static void set_absolute_path(char *options[], const char *option_name,
 #ifdef USE_LUA
 #define main luatest_main
 #define luaL_openlibs lua_civet_openlibs
+struct lua_State;
 extern void lua_civet_openlibs(struct lua_State *L);
 #include "../src/third_party/lua-5.2.3/src/lua.c"
 #undef main