Fix build with Qt 5

<QPrinter> is in QtPrintSupport, so we need to declare we want to use
it.

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface.pro

@@ -6,6 +6,7 @@ QT = core gui network svg concurrent
 lessThan(QT_MAJOR_VERSION, 5) {
 	QT += webkit
 } else {
+	QT += printsupport
 	!android: QT += webkitwidgets webkit
 	android: QT += androidextras
 }