UI: Fix missing files warning icon

The missing files icon would be cut off if the dialog was made
smaller.

UI/forms/OBSMissingFiles.ui

@@ -24,7 +24,7 @@
        <property name="minimumSize">
         <size>
          <width>0</width>
-         <height>20</height>
+         <height>0</height>
         </size>
        </property>
       </widget>