CMake: forward-declare QDialog

Without this forward declaration compiling cmakekitinformation.cpp yields
a compilation error saying QDialog does not name a type.

Change-Id: I55066706e9850a947cac47e6e7150928c2fc6c51
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/cmakeprojectmanager/cmakekitconfigwidget.h

@@ -29,6 +29,7 @@
 
 QT_BEGIN_NAMESPACE
 class QComboBox;
+class QDialog;
 class QLabel;
 class QPlainTextEdit;
 class QPushButton;