warnings: trivial fixes to lib/

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

lib/arp.c

 #include <string.h>
+#include "endpoint.h"
 
 #include "ipv4.h"
 #include "ptpd_netif.h"

lib/bootp.c

 #include <string.h>
+#include <wrc.h>
+#include "endpoint.h"
 
 #include "ipv4.h"
 

lib/util.c

@@ -2,6 +2,7 @@
 #include <stdint.h>
 #include <stdarg.h>
 #include <time.h>
+#include <wrc.h>
 
 #include "util.h"