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/ipv4/etharp.c

@@ -56,6 +56,10 @@
 
 #include <string.h>
 
+#ifdef LWIP_HOOK_FILENAME
+#include LWIP_HOOK_FILENAME
+#endif
+
 #if LWIP_IPV4 && LWIP_ARP /* don't build if not configured for use in lwipopts.h */
 
 /** Re-request a used ARP entry 1 minute before it would expire to prevent

src/core/ipv4/icmp.c

@@ -51,6 +51,10 @@
 
 #include <string.h>
 
+#ifdef LWIP_HOOK_FILENAME
+#include LWIP_HOOK_FILENAME
+#endif
+
 /** Small optimization: set to 0 if incoming PBUF_POOL pbuf always can be
  * used to modify and send a response packet (and to 1 if this is not the case,
  * e.g. when link header is stripped of when receiving) */

src/core/ipv4/ip4.c

@@ -59,6 +59,10 @@
 
 #include <string.h>
 
+#ifdef LWIP_HOOK_FILENAME
+#include LWIP_HOOK_FILENAME
+#endif
+
 /** Set this to 0 in the rare case of wanting to call an extra function to
  * generate the IP checksum (in contrast to calculating it on-the-fly). */
 #ifndef LWIP_INLINE_IP_CHKSUM