Squashed warning for lua+sqlite code path

mongoose.c

@@ -3981,7 +3981,7 @@ static void prepare_lua_environment(struct mg_connection *conn, lua_State *L) {
 
   luaL_openlibs(L);
 #ifdef USE_LUA_SQLITE3
-  luaopen_lsqlite3(L);
+  { extern int luaopen_lsqlite3(lua_State *); luaopen_lsqlite3(L); }
 #endif
 
   // Register "print" function which calls mg_write()