net.httpserver: Make function local, fixes loading since there is no environment [luacheck]

net/httpserver.lua

@@ -5,7 +5,7 @@ local traceback = debug.traceback;
 local _ENV = nil;
 -- luacheck: std none
 
-function fail()
+local function fail()
 	log("error", "Attempt to use legacy HTTP API. For more info see https://prosody.im/doc/developers/legacy_http");
 	log("error", "Legacy HTTP API usage, %s", traceback("", 2));
 end