Fix -Wodr in GCC LTO builds

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

rtgui/controllines.cc

@@ -28,6 +28,9 @@
 
 using namespace rtengine;
 
+namespace
+{
+
 enum GeometryIndex {
     MO_CANVAS,
     MO_OBJECT_COUNT,
@@ -46,9 +49,6 @@ enum GeometryOffset {
     OFFSET_END = (MO_OBJECT_COUNT + ::ControlLine::END) % ::ControlLine::OBJECT_COUNT,
 };
 
-namespace
-{
-
 /**
  * Returns true if the object matches the offset.
  */