Bug#918293: node-xmpp build depends on node-node-stringprep that is currently not in buster

[Adrian Bunk]

Source: node-xmpp
Version: 0.3.2-3
Severity: serious
Tags: ftbfs buster sid
Control: block -1 by 895029

node-xmpp build depends on node-node-stringprep that is currently
not in buster due to #895029.