missing_network: drop unnecessary definition

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

src/basic/missing_network.h

@@ -23,11 +23,6 @@
 #define IPV4_MIN_MTU 68
 #endif
 
-/* linux/ipv6.h */
-#ifndef IPV6_MIN_MTU
-#define IPV6_MIN_MTU 1280
-#endif
-
 /* Note that LOOPBACK_IFINDEX is currently not exposed by the
  * kernel/glibc, but hardcoded internally by the kernel.  However, as
  * it is exported to userspace indirectly via rtnetlink and the

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>