social/mail_activity_calendar/data/mail_activity_data.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <!--
        Copyright 2016 Odoo SA <https://www.odoo.com>
        License LGPL-3.0 or later (http://www.gnu.org/licenses/lgpl.html).
    -->
    <data noupdate="1">
        <record id="mail_activity.mail_activity_data_meeting" model="mail.activity.type">
            <field name="category">meeting</field>
        </record>
    </data>
</odoo>