path: root/addons/calendar/data/mail_activity_data.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data noupdate="1">
        <record id="mail.mail_activity_data_meeting" model="mail.activity.type">
            <field name="category">meeting</field>
        </record>
    </data>
</odoo>