Fix +f not detecting repeats in case of 1st message, REHASH, 2nd message.

This due to a missing SavePersistentPointer() on the siphash key.
[skip ci]

src/modules/chanmodes/floodprot.c

@@ -196,6 +196,7 @@ MOD_LOAD()
 MOD_UNLOAD()
 {
 	SavePersistentPointer(modinfo, removechannelmodetimer_list);
+	SavePersistentPointer(modinfo, floodprot_msghash_key);
 	return MOD_SUCCESS;
 }