Make libdbd-mysql-perl and friends Recommends instead of strict Depends

The Perl MySQL libraries are only required for a few non-essential
binaries:
  innotop
  mysqlreport
  mysql_convert_table_format
  mysql_setpermission
  mysqlhotcopy

Therefore it is more appropriate to have them as Recommends. This will
also help avoid circular dependencies, as libdbd-mysql-perl itself
depends on the MySQL client library (which could also be provided
my MariaDB).

See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=702280#45