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_cyg_ming.mak b/src/Make_cyg_ming.mak
index 091102e..d767c79 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -624,7 +624,7 @@
 endif
 
 ifeq ($(CHANNEL),yes)
-DEFINES += -DFEAT_JOB_CHANNEL
+DEFINES += -DFEAT_JOB_CHANNEL -DFEAT_IPV6
 endif
 
 ifeq ($(TERMINAL),yes)