Fix rig_list_foreach in testmW2power.c

tests/testmW2power.c

@@ -13,7 +13,7 @@
 
 int nrigs = 0;
 
-int callback(struct rig_caps *caps, rig_ptr_t rigp)
+int callback(const struct rig_caps *caps, rig_ptr_t rigp)
 {
     RIG *rig = (RIG *) rigp;