Automatically link printsupport plugins to static applications.

Add the required printsupport plugins to the QTPLUGIN variable
as is done for the QPA plugin.

[ChangeLog][QtPrintSupport] Made the Qt buildsystem automatically include the
necessary plugins so that static applications can print.

Task-number: QTBUG-29663
Change-Id: I0e2e3b0f25dd5714bd187711c85893926b0c4e85
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>