Fix build of the Qt SQL plugins for MySQL against MariaDB 10.

Recent MySQL code has deprecated the my_bool type, so r469165
added a check for that recent version. MariaDB pretends to be
MySQL, and has an even newer version, which nonetheless hasn't
taken over the drop-my_bool-type change. So my_bool still exists
there. Assume, for now, that MariaDB has my_bool regardless,
to fix incompatibilities reported after PR227813 was closed.

PR: 227813
Reported by: Jashank Jeremy