Avoid a warning about unused variables

Author: heshpdx

libs/librrgraph/src/base/rr_graph_utils.cpp

@@ -140,6 +140,7 @@ vtr::vector<RRNodeId, std::vector<RREdgeId>> get_fan_in_list(const RRGraphView&
     //Walk the graph and increment fanin on all dwnstream nodes
     rr_graph.rr_nodes().for_each_edge(
         [&](RREdgeId edge, RRNodeId src, RRNodeId sink) -> void {
+            (void) src;
             node_fan_in_list[sink].push_back(edge);
         });