set privs only when changed

setting privileges causes mt to write to the player db. this
changes shields so it only sets the interact privilege when
it has actually changed.

mods/ITEMS/mcl_shields/init.lua

@@ -182,8 +182,10 @@ end
 local function set_interact(player, interact)
 	local player_name = player:get_player_name()
 	local privs = minetest.get_player_privs(player_name)
+	if privs.interact ~= interact then
 		privs.interact = interact
 		minetest.set_player_privs(player_name, privs)
+	end
 end
 
 local shield_hud = {}