Implement task #14367: Hooks need a better place to be defined

We now have a #define for a header file name that is #included in every .c file that provides hooks.

src/core/ipv6/ip6.c

@@ -60,6 +60,10 @@
 #include "lwip/debug.h"
 #include "lwip/stats.h"
 
+#ifdef LWIP_HOOK_FILENAME
+#include LWIP_HOOK_FILENAME
+#endif
+
 /**
  * Finds the appropriate network interface for a given IPv6 address. It tries to select
  * a netif following a sequence of heuristics:

src/core/ipv6/nd6.c

@@ -64,6 +64,10 @@
 
 #include <string.h>
 
+#ifdef LWIP_HOOK_FILENAME
+#include LWIP_HOOK_FILENAME
+#endif
+
 #if LWIP_IPV6_DUP_DETECT_ATTEMPTS > IP6_ADDR_TENTATIVE_COUNT_MASK
 #error LWIP_IPV6_DUP_DETECT_ATTEMPTS > IP6_ADDR_TENTATIVE_COUNT_MASK
 #endif