Fix -Wodr in GCC LTO builds

I've tested GCC 10 and 11, which exhibit the problem, and 8, which
doesn't.

rtgui/spot.cc

@@ -29,6 +29,9 @@
 using namespace rtengine;
 using namespace rtengine::procparams;
 
+namespace
+{
+
 enum GeometryIndex {
     MO_TARGET_DISK,
     MO_SOURCE_DISC,
@@ -47,6 +50,8 @@ enum GeometryIndex {
     VISIBLE_OBJECT_COUNT
 };
 
+}
+
 Spot::Spot() :
     FoldableToolPanel(this, "spot", M ("TP_SPOT_LABEL"), true, true),
     EditSubscriber(ET_OBJECTS),