Fix issue on re-titling sdbSizer button on GTK.

While "fixes" isn't quite the right word, we do want this commit
associated with the following bug, and "fixes" is the easiest
way to do that:

Fixes: lp:1712579
* https://bugs.launchpad.net/kicad/+bug/1712579

pcbnew/dialogs/dialog_plot.cpp

@@ -56,6 +56,7 @@ DIALOG_PLOT::DIALOG_PLOT( PCB_EDIT_FRAME* aParent ) :
     m_sdbSizer1OK->SetLabel( _( "Plot" ) );
     m_sdbSizer1Apply->SetLabel( _( "Generate Drill Files..." ) );
     m_sdbSizer1Cancel->SetLabel( _( "Close" ) );
+    m_sdbSizer1->Layout();
 
     m_sdbSizer1OK->SetDefault();