MySQL ports: Extend CONFLICTS_INSTALL in conflicting ports.

databases/mysqlwsrep56-server has been present for some time and
databases/mysqlwsrep57-server has been added recently. Both ports
conflict with other MySQL server ports and this has been reflected
in databases/mysqlwsrep5[67]-server/Makefile but must also be
reflected in all conflicting ports' Makefiles as well.

The conflicting ports are:
databases/mariadb100-server
databases/mariadb101-server
databases/mariadb102-server
databases/mariadb55-server
databases/mysql55-server
databases/mysql56-server
databases/mysql57-server
databases/mysql80-server
databases/percona55-server
databases/percona56-server
databases/percona57-server

There is not going to be mysqlwsrep*-client* ports (because the vanilla
mysql*-client* ports suffice), so the entry in CONFLICTS just reads
mysqlwsrep*.

PR: 220791
Submitted by: vd
Approved by: brnrd, ale (maintainer timeout), feld (maintainer timeout), flo, mmokhi, koobs