driver: make luaL_pushnull() non-static

Tarantool will make this method public in:
tarantool/tarantool@878c0d0

This patch fixes compilation error:

./mysql/driver.c:53:1: error: static declaration of ‘luaL_pushnull’ follows non-static declaration
   53 | luaL_pushnull(struct lua_State *L)
      | ^~~~~~~~~~~~~

mysql/driver.c

@@ -49,7 +49,7 @@ static int luaL_nil_ref = LUA_REFNIL;
  * Can be used as replacement of nil in Lua tables.
  * @param L stack
  */
-static inline void
+void
 luaL_pushnull(struct lua_State *L)
 {
 	lua_rawgeti(L, LUA_REGISTRYINDEX, luaL_nil_ref);