Add MySQL plugin to Windows

Added options:

 win: -plugin-sql-mysql

Task-number: QTBUG-29802
Change-Id: I09f78129cec8afaf2c42ac187c84f93cb847d2cf
Reviewed-by: Kimmo Leppälä <kimmo.leppala@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
mkqt5bld: Remove some doc files from installers

Adding *.index and *.qhp files to the list of files
to be removed from the installers.

Change-Id: I94beb16c253d6544f84511c039d38f3b7d65a278
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>