Fix compile error at GCC11

Fixes #3273

Signed-off-by: HungMingWu <u9089000@gmail.com>

docs/CONTRIBUTORS

@@ -31,6 +31,7 @@ Graham Rushton
 Harald Heckmann
 Howard Su
 Huang Rui
+HungMingWu
 HyungKi Jeong
 Iru Cai
 Ivan Vnučec

src/V3TSP.cpp

@@ -141,7 +141,7 @@ private:
     }
 
     struct EdgeListCmp final {
-        bool operator()(const EdgeList* ap, const EdgeList* bp) {
+        bool operator()(const EdgeList* ap, const EdgeList* bp) const {
             // Simply compare heads
             return edgeCmp(bp->back(), ap->back());
         }