Fix no-ipv6
From https://github.com/brndnmtthws/conky/issues/1033#issue-748414180

diff '--color=auto' -aur a/src/linux.cc b/src/linux.cc
--- a/src/linux.cc	2019-08-12 14:53:54.000000000 -0700
+++ b/src/linux.cc	2020-11-22 18:44:04.394329722 -0700
@@ -667,6 +667,7 @@
   }
 }
 
+#ifdef BUILD_IPV6
 void update_ipv6_net_stats() {
   FILE *file;
   char v6addr[33];
@@ -731,6 +732,7 @@
 
   fclose(file);
 }
+#endif /* BUILD_IPV6 */
 
 /**
  * Parses information from /proc/net/dev and stores them in ???