box: make box.iproto available before box.cfg

box.iproto table with iproto features and constants was exported to Lua
in commit fe89aabe ("box: export IPROTO constants and features to
Lua").

Add the table to the whitelist of what's available even before box.cfg.

Prerequisite #5272
Closes #8053

NO_DOC=intermediate state wasn't released, no changes necessary
NO_CHANGELOG=see NO_DOC
NO_TEST=used in next commit's tests

src/box/lua/load_cfg.lua

@@ -861,6 +861,7 @@ local box_cfg_guard_whitelist = {
     txn_isolation_level = true;
     NULL = true;
     info = true;
+    iproto = true;
 };
 
 -- List of box members that requires full box loading.