patch 8.2.0557: no IPv6 support for channels

Problem:    No IPv6 support for channels.
Solution:   Add IPv6 support. (Ozaki Kiichi, closes #5893)
diff --git a/src/config.h.in b/src/config.h.in
index deeceb7..166724c 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -438,6 +438,9 @@
 /* Define if we have shl_load() */
 #undef HAVE_SHL_LOAD
 
+/* Define if we can use IPv6 networking. */
+#undef FEAT_IPV6
+
 /* Define if you want to include NetBeans integration. */
 #undef FEAT_NETBEANS_INTG