mariadb: Adds default-libmysqlclient-dev

[ludeeus]

Description:

Adds default-libmysqlclient-dev to fix this error during installation of the python package:

    Complete output from command python setup.py egg_info:
    /bin/sh: 1: mysql_config: not found
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-vipuxr5a/mysqlclient/setup.py", line 16, in <module>
        metadata, options = get_config()
      File "/tmp/pip-install-vipuxr5a/mysqlclient/setup_posix.py", line 51, in get_config
        libs = mysql_config("libs")
      File "/tmp/pip-install-vipuxr5a/mysqlclient/setup_posix.py", line 29, in mysql_config
        raise EnvironmentError("%s not found" % (_mysql_config_path,))
    OSError: mysql_config not found

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-vipuxr5a/mysqlclient/

Related issue (if applicable): Fixes #

Checklist (Required):

• The code change is tested and works locally.

[Landrash]

Looks good to me.
Has this been checked so that it doesn't break on Stretch?

[ludeeus]

It has not, will see if I get some time to test that too.