Fix -Wodr in GCC LTO builds

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

rtgui/gradient.cc

@@ -12,6 +12,9 @@
 using namespace rtengine;
 using namespace rtengine::procparams;
 
+namespace
+{
+
 enum GeometryIndex {
     H_LINE,
     V_LINE,
@@ -20,6 +23,8 @@ enum GeometryIndex {
     CENTER_CIRCLE,
 };
 
+}
+
 Gradient::Gradient () : FoldableToolPanel(this, "gradient", M("TP_GRADIENT_LABEL"), false, true), EditSubscriber(ET_OBJECTS), lastObject(-1), draggedPointOldAngle(-1000.)
 {