Make nuttcp build with IPv6 support.

Author: Chris Taylor <ctaylor@debian.org> 
--- a/Makefile
+++ b/Makefile
@@ -6,7 +6,7 @@ OPT = -g -O2
 CFLAGS = $(OPT) $(NOIPV6)
 LIBS = 
 ifneq ($(NOIPV6),)
-APPEXTV6=-noipv6
+APPEXTV6=
 endif
 APPEXT = $(APPEXTV6)
 
