Fix warning in USD curves exporter

Use timecode.GetValue() to fix warning about passing correct 'double' type to 'WM_reportf'

Co-authored-by: DESKTOP-ON14TH5\Sonny Campbell <sonny.campbell@unity3d.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/108008

source/blender/io/usd/intern/usd_writer_curves.cc

@@ -437,7 +437,7 @@ void USDCurvesWriter::do_write(HierarchyContext &context)
                "%s on frame %f",
                IFACE_(first_frame_curve_type_name),
                IFACE_(current_curve_type_name),
-               timecode);
+               timecode.GetValue());
     return;
   }