Fix name of widget, it must be unique

The widget name was duplicated, it caused no real issue, sinc it was
never referenced, but the UI compiler complained

sdrgui/gui/spectrummarkersdialog.ui

@@ -510,7 +510,7 @@ Max - Marker will move according to the maximum power at the marker frequency</s
       <attribute name="toolTip">
        <string>Waterfall markers</string>
       </attribute>
-      <widget class="QWidget" name="layoutWidget">
+      <widget class="QWidget" name="layoutWidget_1">
        <property name="geometry">
         <rect>
          <x>0</x>