Change spacer minimum height hint

This changes the minimum height hint of the spacer between
the Status fields and the Poll and Error fields. This seems to
fix the Lock Wallet button disappearing when the window is resized
to the minimum.

Author: jamescowens

src/qt/forms/overviewpage.ui

@@ -413,7 +413,7 @@
          <property name="sizeHint" stdset="0">
           <size>
            <width>20</width>
-           <height>10</height>
+           <height>40</height>
           </size>
          </property>
         </spacer>