DolphinQt: Fix mouse lock checkbox appearing when it shouldn't

See https://bugs.dolphin-emu.org/issues/13232; this was introduced in 7dde0c3c31. Apparently, providing a parent for a widget that is not visible makes your new widget visible when the parent is later made visible, in addition to managing the deletion of the widget; the documentation does not specify this (only that if the parent is visible you need to explicitly show it).

Source/Core/DolphinQt/Settings/InterfacePane.cpp

@@ -204,6 +204,8 @@ void InterfacePane::CreateInGame()
   groupbox_layout->addWidget(mouse_groupbox);
 #ifdef _WIN32
   groupbox_layout->addWidget(m_checkbox_lock_mouse);
+#else
+  m_checkbox_lock_mouse->hide();
 #endif
 }