data/ui/patch-dialog.ui: Fix map-without-parent warning

If the dialog is set to visible in the builder file, it will get mapped
before we have set our parent on it, resulting in an annoying warning.

Author: kaiw

data/ui/patch-dialog.ui

@@ -3,7 +3,7 @@
 <interface>
   <requires lib="gtk+" version="3.6"/>
   <object class="GtkDialog" id="patchdialog">
-    <property name="visible">True</property>
+    <property name="visible">False</property>
     <property name="can_focus">False</property>
     <property name="border_width">12</property>
     <property name="title" translatable="yes">Format as Patch</property>