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>

src/libsystemd-network/ndisc-option.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
+#include <linux/ipv6.h>
 #include <netinet/icmp6.h>
 
 #include "dns-domain.h"

src/network/networkd-setlink.c

@@ -4,6 +4,7 @@
 #include <linux/if.h>
 #include <linux/if_arp.h>
 #include <linux/if_bridge.h>
+#include <linux/ipv6.h>
 
 #include "missing_network.h"
 #include "netif-util.h"

src/resolve/resolved-manager.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 
 #include <fcntl.h>
+#include <linux/ipv6.h>
 #include <netinet/in.h>
 #include <poll.h>
 #include <sys/ioctl.h>

src/shared/conf-parser.c

@@ -2,6 +2,7 @@
 
 #include <errno.h>
 #include <limits.h>
+#include <linux/ipv6.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>