Do not compile with IPv6 support in fabric.p4 by default

Change-Id: I5194ccb0ab78ec4388106cea14c96884f5d16b0f
diff --git a/pipelines/fabric/src/main/resources/include/control/next.p4 b/pipelines/fabric/src/main/resources/include/control/next.p4
index 2fbb866..e0ebbe8 100644
--- a/pipelines/fabric/src/main/resources/include/control/next.p4
+++ b/pipelines/fabric/src/main/resources/include/control/next.p4
@@ -139,9 +139,11 @@
                 if(hdr.ipv4.isValid()) {
                     hdr.ipv4.ttl = hdr.ipv4.ttl - 1;
                 }
+#ifdef WITH_IPV6
                 else if (hdr.ipv6.isValid()) {
                     hdr.ipv6.hop_limit = hdr.ipv6.hop_limit - 1;
                 }
+#endif // WITH_IPV6
             }
         }
         hashed.apply();