Add MySQL plugin to Linux

Added options:

  lin: -plugin-sql-mysql

Task-number: QTBUG-29802
Change-Id: If7a8b515a1d677db3b24a697463550e3ca9c44df
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>