qt5-mysql-plugin: add variant for mariadb 10.2

[ctreleaven (Craig Treleaven)]

Please add a variant to allow qt5-mysql-plugin to access MariaDB v10.2 databases. I believe all that is required is to add the following block around line 809 in the qt5 portfile:

        {
            "mariadb10_2"
            "port:mariadb-10.2"
            "${prefix}/include/mariadb-10.2/mysql"
            "${prefix}/lib/mariadb-10.2/mysql"
            "-lmysqlclient_r"
            ""
        }

I'm working on a port for MythTV 29.x and I would like to upgrade to MariaDB 10.2 and as such need this variant.

Why 10.2? Lacking a current port for MariaDB 10.3, 10.2 is the latest we offer. Also, MariaDB 10.4 suffers from a bug that causes crashes when used with MythTV.

[MarcusCalhoun-Lopez (Marcus Calhoun-Lopez)]

This was fixed in ​https://github.com/macports/macports-ports/commit/28c2cf4d8c5ed14c07513da72d9b1e53536c821c.