Update postprocessingsettingswidget.ui

Fixed an issue where the window of post-processing suddenly jumped to the center after switching filters. (special thanks to user 2048 from ppxclub.com)

src/duckstation-qt/postprocessingsettingswidget.ui

@@ -108,6 +108,19 @@
      </widget>
     </widget>
    </item>
+   <item>
+    <spacer name="verticalSpacer">
+     <property name="orientation">
+      <enum>Qt::Vertical</enum>
+     </property>
+     <property name="sizeHint" stdset="0">
+      <size>
+       <width>20</width>
+       <height>0</height>
+      </size>
+     </property>
+    </spacer>
+   </item>
   </layout>
  </widget>
  <customwidgets>