neighbor/smbclient: add splice_after() fallback for gcc 4.6

src/neighbor/plugins/SmbclientNeighborPlugin.cxx

@@ -215,7 +215,14 @@ SmbclientNeighborExplorer::Run()
 			prev = i;
 		} else {
 			/* can't see it anymore: move to "lost" */
+#if defined(__clang__) || GCC_CHECK_VERSION(4,7)
 			lost.splice_after(lost.before_begin(), list, prev);
+#else
+			/* the forward_list::splice_after() lvalue
+			   reference overload is missing in gcc 4.6 */
+			lost.emplace_front(std::move(*i));
+			list.erase_after(prev);
+#endif
 		}
 	}