missing_network: drop unnecessary definition

Now we have ipv6.h, hence the definition is not necessary anymore.

src/basic/parse-util.c

@@ -2,6 +2,7 @@
 
 #include <errno.h>
 #include <inttypes.h>
+#include <linux/ipv6.h>
 #include <net/if.h>
 #include <stdio.h>
 #include <stdlib.h>