patch 8.2.0591: MS-Windows: should always support IPv6

Problem:    MS-Windows: should always support IPv6
Solution:   Add build flag. (Ozaki Kiichi, closes #5944)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index fa14c9e..e95a94a 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -467,7 +467,7 @@
 !if "$(CHANNEL)" == "yes"
 CHANNEL_PRO	= proto/channel.pro
 CHANNEL_OBJ	= $(OBJDIR)/channel.obj
-CHANNEL_DEFS	= -DFEAT_JOB_CHANNEL
+CHANNEL_DEFS	= -DFEAT_JOB_CHANNEL -DFEAT_IPV6
 
 NETBEANS_LIB	= WSock32.lib Ws2_32.lib
 !endif