mariadb: install staging libs to lib/mysql as well

mariadb: install staging libs to lib/mysql as well

Historically programs added /usr/lib/mysql to the library search path when
linking. mariadb does not do so anymore, although even its mysql_config
script suggests to use this path. When an old version of the library still
resides in this directory (like from an old mysql installation) it will be
linked against instead of the current library which is in /usr/lib.

To prevent this simply install symlinks from the staging libs in /usr/lib
into /usr/lib/mysql. This fixes build failures on the build bots which are
seen with libzdb and libdbi-drivers.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>