Preset include directory for configuration files below MariaDB 10.5 is
/etc/mysql/conf.d

Change installation location wrong plugin installation location from
/etc/mysql/mariadb.d to default include directory /etc/mysql/conf.d.
Change makes gssapi-server, oqgraph, rocksdb and tokudb plugins
loading work after installation

NOTE TO MERGERS: This commit should be upstream to MariaDB 10.4 only!
Merging to MariaDB 10.5 and above leads to major problems.