[Part::Mirror] fix bug where selection gate was not getting removed on cancel button clicked

src/Mod/Part/Gui/Mirroring.cpp

@@ -370,4 +370,9 @@ bool TaskMirroring::accept()
     return widget->accept();
 }
 
+bool TaskMirroring::reject()
+{
+    return widget->reject();
+}
+
 #include "moc_Mirroring.cpp"

src/Mod/Part/Gui/Mirroring.h

@@ -67,6 +67,7 @@ public:
 
 public:
     bool accept() override;
+    bool reject() override;
 
     QDialogButtonBox::StandardButtons getStandardButtons() const override
     { return QDialogButtonBox::Ok | QDialogButtonBox::Cancel; }