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/evalfunc.c b/src/evalfunc.c
index 7170c0e..8ce8c87 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -3943,6 +3943,13 @@
 #endif
 		},
 	{"insert_expand", 1},
+	{"ipv6",
+#ifdef FEAT_IPV6
+		1
+#else
+		0
+#endif
+	},
 	{"job",
 #ifdef FEAT_JOB_CHANNEL
 		1