fix build:jitsi scripts crash caused by a missing folder

On a freshly install of the developer environment, the build:jitsi try
to create a file in ./webapp with the cURL command. However, ./webapp
folder doesn't exist and the build script crash. This patch makes sure
the appropriate folder is created if it doesn't already exist

Signed-off-by: Danny Colin <contact@dannycolin.com>

scripts/build-jitsi.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [[ ! -f "./webapp" ]]; then
+  mkdir "./webapp"
+fi
+
+curl -s https://jitsi.riot.im/libs/external_api.min.js > ./webapp/jitsi_external_api.min.js