[Warnings] Fixed Incorrect Forward Declaration

In RoutePathManager, the forward declaration of the RoutingContext was
incorrectly made a class instead of a struct. This was causing several
warnings in the Clang build (but not in the GCC build). Fixed since the
forward declaration should match the declaration.

Author: AlexandreSinger

vpr/src/route/route_path_manager.h

@@ -28,7 +28,7 @@ struct t_heap_path {
 };
 
 // Forward declaration of RoutingContext needed for traceback insertion
-class RoutingContext;
+struct RoutingContext;
 
 /* A class to manage the extra data required for RCV
  * It manages a set containing all the nodes that currently exist in the route tree
@@ -117,4 +117,4 @@ class PathManager {
     std::set<RRNodeId> route_tree_nodes_;
 };
 
-#endif
+#endif