fixed error reported by clang

crystal.cpp

@@ -555,6 +555,8 @@ struct hrmap_crystal : hrmap_standard {
     exit(1);
     }
 
+  transmatrix adj(cell *c, int d) override { return hrmap_standard::adj(c, d); }
+
   void draw() override {
     if(!crystal3()) { hrmap_standard::draw(); return; }
     sphereflip = Id;