QbsPM: Fix memory leaks

Detected by valgrind

Change-Id: I32008cb91b5b814f67896ad1efb08f1366a38c76
Reviewed-by: Christian Kandeler <christian.kandeler@theqtcompany.com>

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -272,6 +272,11 @@ QbsCleanStepConfigWidget::QbsCleanStepConfigWidget(QbsCleanStep *step) :
     updateState();
 }
 
+QbsCleanStepConfigWidget::~QbsCleanStepConfigWidget()
+{
+    delete m_ui;
+}
+
 QString QbsCleanStepConfigWidget::summaryText() const
 {
     return m_summary;