UI: Make sure size-specific spacers are fixed

Fixes the general settings pane on mac

UI/forms/OBSBasicSettings.ui

@@ -240,6 +240,9 @@
                      <property name="orientation">
                       <enum>Qt::Horizontal</enum>
                      </property>
+                     <property name="sizeType">
+                      <enum>QSizePolicy::Fixed</enum>
+                     </property>
                      <property name="sizeHint" stdset="0">
                       <size>
                        <width>170</width>